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

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.

I've experimented with cables over the years, and have come to the same conclusion that the founder of Monster Cable finally fessed up to: expensive cables are a gimmick conjured up by marketers who figured out that audiophiles will pay a lot of money - even an insane amount of money - for wire (those with wife problems need look no farther than what can be wasted on speaker wire). IMHO, the discussion around interconnects are akin to a theological cult. 

Unless they are are really cheap wire bought at a hardware store, it's not gonna amount to a hill of beans. The Meineke muffler slogan applies here: "I'm not gonna pay a lot for this speaker (interconnect, etc) cable!"

If it was Crazy Eddie's idea of insane prices, that would be a different story!!!

Biggest difference and improvement came with my own DIY pure silver cables -- less than $300 -- no terminations.

Klee Acoustics replaced my Kimber Kable Monocle XL’s...By comparison... the Kimbers sounded broke... ie; closed in,lacking in image,sound-staging,resolution, and tone. Of course,this is with my ears and my equipment. ymmv