I've not once had a bad MTX sub, I've had 5000's 6000's and know a few people who ran 8000's they're good solid products. The amps are now Xtant, or Xtant is now MTX or some crap but anyways Xtant were great SQ amps and the MTX amps used to be on par with the old Fosgates. There's nothing wrong with MTX products that I've seen though they change there product lineup so often it's highly possible to get a bad design somewhere in there.
I work at Circuit City so I hear MTX's all day long. The 5500 I used to like, but now I hate it. I have found that 1 7500 in a ported box can actually get pretty loud, low, and sound good. When I hear the 2 7500's with more power to them though, I don't like the sound. To me the Kicker CVR12's (and my RE's) blow the MTX's out of the water.