If you're talking about speaker level, then yes there is a difference IMO. For other kinds of cable, the only difference I can hear worth noting is the changes in the sound and drop of interference when good shielding is used.
I've done blind tests, and come to the conclusion that any differences in line level cables aren't really there.
It's like I swear I can here a difference between two CDs, both burned in exactly the same way with exactly the same content and exactly the same disc on my Mac. However, when I close my eyes, I realise there really is no difference whatsoever. It's a similar thing with cables, if you didn't know the 'improved' cable was there, I doubt you'd hear a difference.