I once bought a pair of B&W 803 speakers from a dealer (when I was VERY new and ignorant about this hobby). He PROMISED they would sound better than the Boston Acoustics speakers I had, even when plugged into my $600 Denon receiver. Well, they didn't. In fact, the Boston's sounded a tad better. Fortunately for me, he allowed me to return them (which was the agreement before I bought them, since I was skeptical).
I'll bet you my annual salary the B&W 803's were the better sounding speaker (at 10x the cost, the should have been). However, I didn't have the equipment necessary to reveal their capabilities. I've had that same experience in the last two years when adding Bryston equipment (BDA-1, BDA-2, BDP-2, 4B SST-2, etc.) that was an upgrade over my current equipment. Better equipment yielded a better result. However, I was able to appreciate the BDP-2 more when I had (demoed) the 4B-SST2.
I've heard power cables make a change in the sound. By my account, it was a minor change, and it was under ideal circumstances, and not worth the cost increase (in my opinion) for the given improvement.
At the end of the day, if you can give equipment (cables, sources, speakers, etc) a test run in the environment you will use it in, you can best judge whether the difference (if any) is worth the cost (to you).