I hate getting involved with religious debates (although I am in this case), but my own experiences agree entirely with John's assertions about speaker cables. As yet, my system shows no real improvement over 12 gauge wire with any higher end cables that I've tried. However, with respect to interconnects, I've found pretty big differences in many of the low to mid-line cables. In fairness, I haven't tried very many higher end IC's, and it wouldn't surprise me if the differences beyond a certain point were somewhat less than dramatic. However, between cables such as Radio Shack Gold, the MIT Terminator series, and Nordost Blue Heaven (relatively low cost cables), I've found that the differences are clearly audible.
As always, your mileage may vary, and I'm willing to accept that other folks do indeed hear differences that I do not. However, since personal musical enjoyment is my only goal, I'm content to spend a wee bit more on IC's where I do hear a difference, save my money on the speaker cables where I don't, and spend the rest of the time sipping wine and playing music.
Cheers,
Ken
