I recently bought two preamps to test against each other and then keep whichever sounded better. I ordered one of them from Audio Advisor, and while talking to the salesman and explaining what I was doing, he strongly advised that I refrain from testing the two preamps back to back, that each pre would need a few days connected to the cables for the cables to "settle in". I ignored the advise - there is no logic behind it (what exactly takes 48 hours to settle? who won the Nobel prize for discovering the cable-settling effect?). I think that kind of thinking represents the audiophile "fringe of pseudo-scientific madness".
On the other hand, historically I have not placed much value on cables. In fact for a while I thought fancy interconnects were useless - wire is wire, copper is copper. Then a couple of years ago I bought a VPI turntable from SteveWVU, and he threw in an Audioquest Jaguar RCA cable and said to me "this cable sounds horrible, see for yourself”. I had trouble believing it, but every time I have used that cable since I found it rolled off the highs. It could have been expectation bias, but then I loaned the cable to Rev without describing the weakness; he used it for a few months and also came to the conclusion that the cable “sounded bad”. Then last week JimPittsburgh loaned me 4 interconnects claiming they sounded different/better. At least two of those 4 distinctly improved my ability to detect instruments in a recording. One of the two “better” cables is a cheap, long (maybe 5 feet) interconnect that Jim also can’t understand why but it sounds really good. So now I am in the camp that cables can make a difference. Expensive cables can sound bad and inexpensive ones good, price does not automatically guarantee anything.
The capacitor debate may be similar. I have changed a lot of caps, and noticed huge improvements when doing so to vintage equipment, but you would expect that, as the caps you are replacing are old and most probably out of spec [the Paccom caps in old Carver and similar vintage equipment are particularly bad!]. My experience improving the sound of newer speakers by replacing stock caps with more expensive capacitors, however, is so-so. I can sometimes notice a difference, but most of the time not so much. Could be a matter of "learning to listen", as Anders puts it, but that rings subjective to me. I tend to side with the Crite camp in that a cap that measures the desired capacitance and has low ESR should optimize the sound relative to what the circuit was designed to do, whether it was manufactured by Panasonic or Sonicap. That is just my gut sense, in no way am I intending to offend those that believe otherwise (I am sure they are trusting their gut too) - just posing my point of view. Could be that there are additional measurements that need to be considered - distortion introduced by caps made one way vs another, etc. Good discussion in any case!