Dustin, welcome. The speakers have been discussed at enough length already that I'll only comment that, as has been said, it'd be even a bit better if you had the room to use an identical vertical M80 as the center.
As to the amplification, you should understand that the 400 watt maximum stated for the M80 has nothing to do with the power actually required for home listening. The max figure relates to the amount which it theoretically could use(it's actually been tested by Axiom with over 1000 watts)without suffering permanent damage(although the same couldn't be said for your hearing at that sound level). The power actually required would be about 1 watt at a comfortably loud average level of about 85dB at a typical listening distance of about 10 feet. Brief split-second peaks on source material that had a very wide dynamic range(softest to loudest)would use much more, of course, but for safe levels the typical receivers rated anywhere around 100 watts are sufficient to do the job.
You appear to be overly concerned about the impedance rating on a speaker. In order for an amplifier to have an "official" 4ohm continuous power rating(for at least 5 continuous minutes at full power under the FTC regulations)it would have to pass that severe test without overheating and shutting down. Relatively few are designed to do so, but they can nevertheless handle the less severe home listening scenarios without a problem. The fact that an amplifier has a continuous power rating at 8 ohms and possibly also at 6 ohms, but not at 4 ohms, is therefore rarely of any practical significance.
At this point you're considering an Integra model which would do the job as described above, but unless you're able to get an unusual deal on it, an essentially identical Onkyo would be available for less. Rather than the DTR-20.3, a model such as the DTR-40.2 with more advanced features, especially the superior Audyssey MultEQ(rather than 2EQ)should be considered. In most cases an Onkyo 708 or 709 which would meet this requirement would be available at a lower price(than the 40.2).
Enjoy the music, not the equipment.