Cables can be used for system fine tuning. I've tried a few, the most expensive being some €250 silver interconnects. Their most obvious effect, to my ears, is akin to an equalizer. My silver cables brighten the sound and cause emphasized sibilance in my system. I no longer use them. Silver has that reputation, though I don't believe all silver cables sound like that. My very first aftermarket interconnect, by contrast, was dull compared even to the patch leads that came with the equipment, never mind the silver cable. On eventual inspection it looked like a solid core, copper power lead. I had to wonder if that's not exactly what they sold me, albeit in fancy packaging at a marked up price.
In my experience cables don't affect fidelity much, though they can slightly alter the perceived frequency balance and, for example, emphasize the bass or treble. I thought the above-mentioned cable dull, but that could be just the ticket in an overbright system. In general I have found no correlation between price and the perceived benefit (or lack thereof) in my system. Nor is there a correlation between price and, say, greater clarity or treble extension. Some of the more expensive cables I heard seemed deliberately designed to take the edge off the treble, for example. This is the problem with cables. That €500 cable might just be the ticket for you, but so might a €50 one, the odd good-quality free patch lead or, for that matter, a €250 equalizer.
By far the biggest improvement in my own system came with the Audeze LCD-2. Sources, amplifiers and cables are further down the list, with cables coming last. FWIW I find analogue interconnect cables make more of a difference between my DAC and pre-amp, rather than the pre and power-amp.
Unlike Lenni, the cables in my system haven't made much of a difference. Among them are some entry-level Nordost cables. I believe his are more upmarket.
Edited by fred43 - 3/5/12 at 2:15pm