This was precisely my own experience with supports until about two years ago. I couldn't tell much difference at all, and ended up with an Ash designs jobbie bought on dealer recommendation and aesthetic grounds. I think that most of the non-Mana stuff out there does sound pretty similar.

I was very happy with it, and intensely sceptical about Mana. That stance has changed!

Back on topic, I use MusicWorks mains leads which were given to me. I couldn't tell any difference between them and the standard Naim kettle leads, so I've left them in. WM brought some sort of mains lead with him when he visited, but I didn't like it much.

Matching impedences may be important when it comes to power cables, but I don't honestly know for sure.