I'm running an avalanche 15 in 3 cubes and my brother is running an SX 15 in just over 3 cubes....
The SX is louder by a little bit, not much, but hands down the ava is a better sounding driver. I would NEVER choose the sx over the ava, but that's because I'm turning into a SQ guy.
On a side note, I was putting 1000 watts to it for a while and it took it like a champ, if that matters to you.
I will give you more info on the setups and differences if you would like them, just PM me...I'm not on here as much during the summer.