which would give me the biggest improvement in sound.
I'm with Gregm...your speakers are fantastic but when it comes to upgrades you can get more value from a speaker upgrade than the same money spent on cables.
I would add room acoustics to your upgrade list too.
If you want to ignore widsom and stick to spending ever more $1000's on wires/cords then by all means go ahead but this is not, IMHO, "the biggest improvement in sound".