Obsessing over mains filtration and conditioning systems is not part of my life. One of the primary reasons I sunk my life savings into building a dream listening room was to avoid the problems created by noisy, grungy mains. Thus, I did what can only be done when you start from scratch: I specified audiophile-grade mains wiring, the finest consumer units (separated from the house) and AC outlets, PVC trunking - everything which is today accepted as beneficial for mains purity, but which can only be incorporated into a system when you're either re-wiring your home or building a new dwelling. I had assumed the room would then be immune to AC tweaking.

As each and everyone one of you knows, I have been proven wrong abut this on more than one occasion, not the least being shown what can still be achieved by the incorporation of myriad AudioPrism accessories and the use of a Nightingale mains distribution unit with filtering and conditioning. Yes, these do make audible improvements pretty much across the board. But it wasn't until Nic Poulsen, late of Trilogy, turned up with his IsoTek SubStation - configured as he thought I would need it - that I felt such add-ons could fall under the 'utterly indispensable' heading.

'Configured' is the key word because the IsoTek SubStation is not a fixed, off-the-shelf purchase. I'm loathe to say that none of its competitors are prepared to custom-build units to an individual's requirements, but if they are, then they're keeping pretty quite about it. The one Nic provided for this review consisted of the 'full monty' Hybrid version, but with two of the six unswitched AC outlets which fill the back of the 440x140x345mm unit wired for America's 120V; you'd be surprised how much stuff arrives from abroad NOT wired for UK voltages.

In the Hybrid SubStation, Poulsen installs both overkill filter modules and isolation transformers. The standard filter module is a multi-stage compound filter design with transient two-stage suppression between live/neutral, live/earth and neutral/earth, while the isolation modules feature hand-wound EI transformers with Faraday screens. Nic specifies top-quality parts throughout, including internal wiring made with sliver plated PTFE cable and double-sided PCBs mounted on a stainless steel base. All modules are individually fused, with the fuse in the neutral line so that it also protects the transient suppression devices between neutral and earth. The live-to-earth devices are protected by the external fuse located in the IEC socket. An LED mounted on the PCB shows that the module is on and the fuse intact.

As you can imagine, this kind of hardware means that the IsoTek can weigh more than most OTT power amplifiers, which is just what the blue-ish, beautifully-finished unit looks like: a big power amp. It is not something you'll feel the need to hide. But it's not something you'll want to move around any more than necessary.

A basic SubStation starts with a single 150VA module, designed with 'typical source components very much in mind, and items needing more than 75Watts. The larger 300VA module will deal with larger valve pre amps, larger AV components and smaller integrated amplifiers, while the 600VA module should be enough for large integrated amplifiers and medium power amplifiers. (For massive amps of 100W/ch-and-up variety, I still stick with the wall-mounted AC outlets.) The basic SubStation ships with one high quality filtration module for its six outlets.

Nic points out with pride that, 'this basic unit contains three times as many components for dealing with high voltage mains spikes, like lightning strikes, than its closest competitor! But it is its future proof configurability that firmly wipes the floor with the opposition.' Any SubStation can be upgraded to accommodate a maximum of six dedicated filter modules, one for each piece of audio equipment. Whereas most rival ranges consist of what is one basic model sold in differing AC capacities, the SubStation's flexibility is such that you can specify filtration or conditioning or both, of varying levels and - if, like me, you have products which aren't designed for 240V mains - assorted voltages. This is a boon to, say, ex-pats who arrive on these shores with one or two cherished components from Japan or the USA.

Nic also makes a point of telling users that the filter modules 'not only stop noise from entering the load but equally as important, stop noise leaving the load, which is why one filter per load is significantly better than a single filter.' Interestingly, IsoTek recommends that a SubStation contain at least two filter modules for improved performance, and these should be employed to 'separate digital components from analogue components.'

In my system, DACs and CD transports benefited most, the primary audible gain being a unexpected improvement in transparency. I hadn't realised quite how susceptible digital sources were to even minuscule traces of mains-born crud, because - trust me - my room extra filtering enjoys pretty clean mains. The clean-up was not unlike moving from single-ended to balanced operation.

After digital hardware, valve pre-amps and small power amps (solid-state or transistor) showed the greatest gains. Here, the improvements indirectly affected transparency. The actuality was that they simply ran more quietly, with less tube noise; this isn't quite the same as increased transparency, although the net sonic gains are similar. What valve gear gained via the IsoTek was a sense of reduced strain and greater 'security'. Maybe I'm neurotic about running fragile, vintage hardware, but, suffice it to say, I felt more at ease with my elder Radfords when they were plugged into the IsoTek.

IsoTek SubStations start at £395, and fully loaded units will pass the £1000 mark. When you consider a grand will not pay for a meter's worth of any of the top cables on the market today, the SubStation looks like a gift. And it's a whole lot cheaper than building a new listening room. Trust me. And if you guys didn't hate video, I'd tell you how it makes your...