What speaker cable made a major difference in the sound quality of your speakers??

Actinote Aria. I've been through many cables for my Duevel Bella Luna Diamantes and these are my "done" cables.

best ever was Atlas Asimi speaker cable and interconect also asimi . but not in my acceptable price range, i never heard anything like that, in bass and tweeter regions. but price of calble is insane :(
and interconect i managed to get of Asimi line;)
next best is silver HGA x32.
tryied MIT and other cables on my tube rig and tannoy kensington GR speakers, and compared to any silver cable, copper wires always were dry ,dead sounding. so rule how much pay ,that you will get i believe is corect

Tried many cables over the years ... some cheap ... some expensive
Finally found the right fit

Clear Day  Double Shotgun

Been with them for a few years and will not entertain any other

My speaker cables cost more than your price range and were purchased new but honestly they make absolutely no difference at all to the sound. Extremely solid and the highest quality robust connection is all I have observed from using industrial/military/aircraft grade style connections.

If your system is "singing" at its best, then may I suggest (no I'll say urgently suggest) you try Townsend Isolda DCT Speaker Cables. www.townsendaudio.com from here in the UK. They are the best speaker cables I've ever used (and I have tried lots over the years) and when used with Townsend Seismic Podium Speaker Stands you will get a "jaw dropping difference"
The sound is dynamic, has dimension, with clean non boomy bass and is wonderfully relaxed.
With the dollar/pound exchange rate in your favour why not give them (both) a try. You will not be disappointed. BTW, I have no connection with Townsend -- well only my Avalon Eidolon Diamonds.