The 8100D is 1500 at 2 ohms right? If so, either will be a good choice. You might want to have a configuration for the HX2s where they will run at 4 ohms each therefore creating a 2 ohm load, b/c I don't know if either are stable under that. But to answer your question, go with the Rockford. Either one would be great though!!!
yeah neither of those amps is stable to 1 ohm or at least i know the RF isn't. if the MTX is stable to 1 ohm than get the MTX. unless you want to wire them in series to get them 4 ohms each. with that RF amp that's your only choice. RF subs and amps compliment each other, i know people disagree with stuff like that but personally i would stick with a RF amp if your gonna run RF subs. those are just my 2 cents.