I'm more interested in SQ than SPL. Heres the options
For a 2 ohm Infinity sub

MB Quart RAA1000 500rms about $270
Alpine MRD-M605 400 rms@ 12V ~$260
MTX 801D 800 rms $270 ish

or something else?

 

Personally, I'd go with the MB Quart, because it has the capability to for 1 ohm. But putting that much power won't mean you'll get SQ.

And with the right wiring, a low RMS amp can make a system sound excellent.

My suggestion... seeing that you want SQ and not really SPL Alpine amp. But if you're interested in turning it up to get some SPL out of it every once in awhile. Go with the MTX.

:edit: I would suggest my Memphis which is my company of choice. But they can't beat those prices.