I have 2 12 inch MTX jackhammer subwoofers with a max of 800 watts RMS or 1600 watts. What amp should you use to power the system and why?

That depends on a few things. Since your question is about power matching(and to help keep it short and sweet) I'll touch on the thing I consider most relevant. "Are you looking to build an S.Q. system, an all-out SPL system?" If you're looking to build a Sound Quality (SQ) system that won't be played at Earth-shaking volumes, then I'd say you could very easily get away with simply matching the RMS power rating of the speakers. (using sealed enclosures and running them in series) If SPL is what you're after, then I'd suggest an amp capable of AT LEAST 50% higher RMS rating than the speakers. (using ported enclosures and running them in parallel)