I'm probably not the best one to respond because I don't even hear a difference between different interconnects!

But since you asked...

I doubt that 21 feet is noticeably different than 6 feet. Especially since you intend to shield the run.

That would have been my only concern. If it comes down to a choice between longer low level cable or longer speaker wire, I'd go for the longer speaker wire since the low level stuff is rumoured to stand a better chance of latching on to stray signals, hum, etc.

(And for the record, I don't hear a difference between speaker cables either.)

Jeff Mackwood