I can personally vouch for KnuKonceptz wire.. the stuff is ridiclously thick and it's all wire.

 

I recenlty piad $14/m for 1/0 from Acklandfs Granger, then $3.49/ft for the same stuff from Princess Auto when I needed more. It's INCREDIBLY flexible and 100% copper

 

I have been using the same old welding wire for 10 years to run about 1K of amps from 1700cca of batteries plus I have run a 1500w invertor that I have run from it with no issue.

Now in the interest of full disclosure, I don't have a true SQ system as I'm not running funky pups and using magic dust to enhance the transient response along with pure silver speaker wires that the virgin has pissed on for an increased sq. (defined so elegantly by df.dima a few years back) but it sounds OK to me with that welding wire