It probably took a bit longer to grow the wood which forms the Mystery One's case. All I know is that nearly a year has passed since I first took delivery of the prototype, and enough detail changes occurred between 'pre-production' and 'shop-ready' to require the loan of a fresh sample. But it was worth waiting for a pre-amp to match the Unison Research Smart 845.

Whatever the wizards in Italy did to that early Mystery One, as seen at the 1995 Hi-Fi Show and still serving as the demo sample 'on the road', it wasn't a simple retrofit. The rather large, garish metal name-plate on the top has been replaced by one you needn't measure by with surveyor's tools, and the top-plate is now held by four quick-release screws. But those of you who recall the swoopy wood, the long, slender rotary controls and the chic charcoal cover will be pleased to hear that it looks just the same as before: drop-dead gorgeous. Unison Research has cornered the market for sexy woodwork in amplification, in the way the Sonus Faber has the same for speakers, and there are probably enough customers out there who'd covet the Mystery One for its looks alone. So let's get that bit out of the way.

Like every other product to emanate from the Unison Research factory, Mystery One is styled like nothing else on the market. It measures a large 490x430x160mm (WDH, including knobs and earthing post), so an ample shelf is needed. I was pleased to find that it fit perfectly in the Hi-Fi Newsstand, but I suspect that its iconoclastic appearance might cause problems for those hoping to slot it into a space which previously held a metallic, cubist device. Carlo Chiarello has created an organic look consisting of a sculpted, solid cherry frame, with a chunky front panel slotted for both ventilation of the valves and egress of the rotaries. The curve of the end-cheeks lends them carrying handle status and a forward crouch. While a collection of four knobs suggests that you're looking at some kind of hi-fi equipment, the effect is still more in keeping with a modernist's idea of a rustic cigar humidor or jewellery case. Or maybe a horizontal knife rack...

Peering through the upper slot allows you to see the glow of the three double triodes (two ECC82s and one ECC83) and the EZ81 double diode rectifier which form the heart of this Class-A line level pre-amp. The lower aperture houses the controls, the on/off rotary at the extreme left with the trio grouped at the right consisting of the source selector, the tape monitor/source selector and the volume control. At the back, all is conventional: gold-plated inputs for four sources plus tape, outputs for tape and two power amps and an IEC mains socket. The one teensy exception to the norm is a multi-pin connector to power a Simply Phono, Unison Research's £495 optional external phono section. Note, however, that it's only the power source for the phono stage; the phono signal is fed separately to the input marked aux/phono rather than through the multi-pin socket.

Release the four screws to remove to cover and you'll see why this sucker weighs a power-amp-like 15kg. The power supply fills the left-hand third of the case, the rest of the space occupied by a motherboard containing a daughterboard for the EZ81 rectifier. The socketry is soldered directly to the main PCB, as are all of the main components, the other three valve bases and the volume control. The source and tape selectors are connected to a second daughter board at the back of the main PCB. Despite the presence of secondary PCBs, the layout is tidy and the build quality impossible to criticise.

It's a clean, minimalist design making use of the extra acreage by spacing the components as far apart as is beneficial, preventing unwanted interaction without adding too much length to the signal path. Then again, the PCB traces are wide and substantial, so signal fragility doesn't seem to be a by-product of the longer path. And then you see the secret weapon, designer Giovanni Sacchetti's little twist: a row of four CR2032, 3V lithium batteries providing the pure DC supply for the valve grids.

Fear not: these aren't obscure cells but 10p-piece-sized buttons available from any camera shop. Apparently, they'll last for something like 5-10 years, so you'll be replacing the valves before you need new batteries. Sacchetti chose this method because it provides superior frequency linearity, and I suspect it plays some part in making this one of the most quiet valve pre-amps I've ever used. The Mystery One is described as having constant input and output impedance for ideal matching to most source components and power amplifiers, gain is 21dB and the signal-to-noise ratio better than 90dB. Other specifications include output impedance of 800 ohms, THD of less than 0.1 percent, maximum input voltage of 8V peak to peak and maximum output voltage of 80V peak to peak.

Its natural partners are the Smart 845 monoblocks. And not just because they both wear cherry trim. At £1750, the Mystery One is priced just right for pairing with its £3000 single-ended siblings, and the two work together as synergistically as you'd expect of a pre/power combo emanating from the same designer. What was unexpected was faultless synergy with the GRAAFiti 5050, which shares with the Mystery One only an Italian valve lineage. But come to think of it, I couldn't cook up a single mismatch, no matter how hard I tried.

In the course of a year, with two different sample, I tried the Mystery One with solid-state power amplifiers as varied as the Sutherland 2000 monoblocks, the Acurus 250 and the Monrio HP1. Tubular mates included the aforementioned Latinate devices, plus the McIntosh MC275, Audio Note's Conquests, the refurbished Dynaco Stereo 70, the Quad IIs (original and reissue) and a bunch of others. Why so many? Partly because I kept the Mystery One within easy reach the entire year. And partly because I fell in love with it.

It's easy to do. The Mystery One is 'classic valve' in all the right places, yet it's as modern as it needs to be in terms of handling ultra-quiet digital source components. CD can't fluster it; neither, for that matter, does open-reel tape. While I didn't have Unison Research's own phono stage -- I used the E.A.R. 834 and Audio Alchemy's VAC-In-The-Box -- the Mystery One is more than sympathetic to analogue signals. What's consistent about its behaviour from source type to source type, from amp to amp, is its 'easy listenability'. Which should explain why I was prepared to clock up so many hours with the Mystery One, despite access to a dozen other pre-amps at any given time.

You just switch it on, leave it for ten or fifteen minutes, and return to enjoy some of the sweetest sounds imaginable - but without making any excuses for its tube innards. With a facility I've seen demonstrated repeatedly only by Audio Research and GRAAF, Unison Research has endowed the Mystery One with the kind of operational composure, speed and background silences usually associated with solid-state pre-amplifiers.

Now let's not get all sarcastic and suggest that such abilities/qualities are or should be anathema to tube lovers. Why not admit that we're talking about three particular virtues normally sacrificed by tube lovers in place of warmth, natural vocals and a sense of space? As far as I can tell, the open-minded designers - of both tube and transistor gear - always strive for a balance consisting of the sonic virtues of both technologies. Or am I imagining the number of solid-state hardware manufacturers who tout their gear as sounding 'tube-like'?

On the other hand, I never hear about tube product manufacturers boasting that their wares deliver transistor-like performance. Either way, the valve designers have to approach sound from the other direction, without cutting their own throats by making tube equipment which sounds solid-state. Whatever anyone tells you, I've yet to see 'solid-state sound' hyped as virtuous, only solid-state abilities, like cool running and freedom from microphony. The Mystery One comes as close to juggling the virtues of both technologies better than just about any other pre-amp this side of an Audio Research Reference One.

Face it: whatever bleating one cares to issue about the glories of the valve, we've all had to live with digital signals for so long that we're conditioned to expect our sounds to emanate from a truly silent background. Anything else is masochism (including my perverse penchant for pre-Dolby tape hiss). So let's not confuse nostalgic worth for sonic superiority. The Mystery One sounds like a vintage tube pre-amp, if by that you mean noisy and cloudy and suffering from aged components nearing total break-down.

Aside from its silent 'canvas' and complete freedom from fatigue-inducing nasties, the Mystery One has other virtues which make it so appealing. Not least is the recreation of a soundstage so vast - 'cavernous' would be the perfect word if that didn't also imply 'echo-ey' - that it's a great way to test a speaker's ability to disappear. With both psuedo point sources and dipoles, the Mystery One fed a seamless left-right/front-back image that seemed to defy the capabilities of the speakers. In this respect, it begs the collaboration of partnering components like old Denon m-c cartridges and LS3/5As. Maybe the 'mystery' in the name refers to its disappearing act.

Yet again, I've found another pre-amp which favours vocals to a point where you might wonder how much warmth and emotion one is allowed to enjoy before being accused of unbridled anachrophilia. But the warmth is never an additive, never a coloration. Think Sonus Faber Minima Amator, or California Audio Labs CD players, and you'll understand the significance of such a finely balanced midband.

Where the Mystery One might disappoint is in 'force'. It lacks aggression and exhibits so much finesse that it seems incapable of letting loose. Yes, gang: the Mystery One is not the first choice for headbanging. It's all a case of perception, I know, but this pre-amp is so refined and polite that it borders on the complaisant. To put it another way, and without wishing to suggest that he's even remotely obsequious, this is the tube pre-amp that Stan Klyne would make if he ever abandoned trannies.

But an excess of is a small price to pay, especially when you get your tube characteristics AND most of the worthy solid-state virtues, too. The real mystery, then, is how did Unison Research do it...