We also have 14 gauge stranded copper wire that is used in our Servo sub kits that should work well for the woofers of the LS6s and will be a lot more flexible than solid 12 gauge wire. It's a little more expensive at $1 per foot, vs the 80c per foot of the standard internal cable.
We're considering using it in our NX-Otica/NX-Treme kits for the 4/8 woofers arrays, keeping the 16 strand for the mids and tweeter circuits