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

Buy fine round dead soft silver (not sterling) from riogrande.com, stick it in a thick untreated cotton rope, add TechFlex on outside and don't use termination for speakers or amps. Bigger is better! Don't go thinner than 12 gauge - 10 or 8 gauge is better - depending on budget. Make sure you're buckled up, when you turn on your system ;-)
Decibell

Clear Day is more in your face than SP7, imagine most listening to rop/rock
would prefer Clear day, someone listening to a symphony the SP-7 .
Of course SP-7  costs much more .

All round the AudioArt SC-5e is better than either , at a few bucks more than Clear Day .

None. I use ESL's, which require a series resistor for tuning the step-up transformer. So I removed the resistor and inserted Nichrome resistance wire, of the same total resistance, between the mono blocks and the speakers.

Essentially, I substituted the highest class resistor for a second class resistor, and have no 'cables' whatsoever. These non-existant cables are very transparent, I have to say.

@falconquest , you don't have to purchase the Clear days. Just talk to Paul at Clear Day. He will let you know which configuration will work better in your system. Then he will send you the cables, without taking your credit card information. Audition the cables, and if you don't like them, simply return them back to Paul. Good luck in your search.

@schubert
My experience between the Clear Day Double Shotgun vs. the Morrow SP7 runs different than yours, likely due to different systems, I'm guessing.  I found the SP7's and the Clear Day's both to provide a somewhat recessed presentation, but the SP7 just sounded mechanical and phasey in the upper mids/low treble in my system at the time (EAR 890 driving Vandersteen Quatro's).  Mike Morrow generously doubled my trial time to allow for complete cable break in, something he said these really required.  I ran them in with music playing through them for easily over 700 hours, but I still found them wholly uninvolving, and even weird sounding.  I will write it up to a "system dependent" interaction issue.

For the record, I listen to the full range of music from classical symphonies and chamber music to jazz, to rock and EDM and everything in between.  The Clear Day's definitely sounded better to my ears, and that is what I bought from this shootout (even after Mike offered to sell the cables to me at 1/2 of the 30% off price that I originally was to pay).

The nice thing is that both Morrow and Clear Day allow for home audition before committing, so why not try them?