Speaker Cables For Big Horns & Tubes

+1 for Tempo Electric simple solid silver conductor cables.  I use between my Coincident Franky Mk-II 300B's to 104dB horns.  Excellent combination and very affordable < $400. 

" Sure. Use 12 gauge OFC cables. The cost should be about 50 cents a foot... "
  100% correct. If it makes you feel better to spend gobs of cash and slurp up bad advice spend many thousands and have at it. Before you do though conduct a true blind test with what you are considering against 12g zip cord and see for yourself.
  " The amp is adjusting to the wire not other way around. "  Curious to know how an amp does this. Please be specific in your explanation of how this occurs.

Purist Audio Cables highly recommended. They are costly but tend to sound smoother, fatigue-free and have a quiet background.