matching amps w/ subs

You have a bandpass box.  These boxes create a frequency region "bandpass" where the SPL output is very high.  However, above or below this region the output drops substantially and when the subwoofer tries to create ultra low frequencies, those below the lower tuning frequency of the box, it will distort, bottom out, sound like ass, whatever you want to call it.  A 30hz high pass filter will start substantially taking out the frequencies below 30hz.  Since your subwoofer won't try to reproduce these low frequencies it has a hard time with, your bass will sound better playing what your box was meant to play. 

Well, the MTX amp probably actually puts out what it says it does.  MTX is made by the same company (Mitek) as Coustic.  Coustic is the "value" brand compared to MTX.  Within that company the brand hierarchy goes Coustic < MTX < Xtant.  Coustic is better than Boss or Riot, but either way the MTX is going to be more honestly rated, tough, clean, and long lasting.  It is simply a better amp than those others.  Trust me, it is not worth going with a cheaper amp that "says" it does 600w or 850w. 

Any thoughts about a Hifonics Brutus BX1000D?

Way more power than you need for that sub/enclosure combination.