Help with upgrading speaker cables

Wow,
Thank you to everyone for your suggestions. I will do research on the cables mentioned and work from there.
Thanks again for all your help.

Another question: I assume performance diminishes with time with cables. Have anyone seen this happen in their system?

No technology diminishes with time the cables are just aged( like wine ?)

Yes a cable can make your sound worse. Absolutely, some cables are just a poor match.
'BTW how much do those Synergistic cables you like cost. Please be honest I would like to try them if they aren't real expensive. Please don't ask what expensive is because it depends on how much money I have at any given time.

JPS Superconductor 3 speaker cables would seem to fit your description. They have constituted the single, most wonderfully sounding cable change ever IN MY SYSTEM. I find them more natural sounding, musical and listenable/relaxing than both the Synergistic Research Tesla Accelerator and Acoustic Reference.