You really can't go too big on wire. Your sub may not be capable of handling that high of a bass, subs aren't meant to play words, it also could be the enclosure, either improper size or air leaks, etc. Make sure your gains are set correctly, setting them too high can make the voltage rails sag, causing clipping (extreme distortion). Bad RCAs or ground can cause this too, as Jesse said.
Yep. Crossover problems, especially with very narrow slopes like 6db/oct. although this isn't as common a problem as wiring, enclosure, and clipping. Depends on the amp.