I'll second all of the alternate sub vendors that have been mentioned (HSU/Axiom/SVS/eD/Outlaw). Three months ago I upgraded to the Outlaw LFM1 Plus, very very pleased so far. Let's toss in Aperion as well. I've heard their Bravus 8D and will say it's a little dynamo...size does not matter.

FireGuy
M22V3/M3ti/M2V3/Omega Super 5/Aperion 5C/OutlawLFM1+Sub/Denon AVR1906/YamDVDC750/AS AMP110