I got an AVR X3700H and a Monolith 5x200 a couple of months ago and they work perfectly together. The Monolith is built like a tank and runs very cool, I’m pretty sure the 2x would be the same. That said, there are a lot more amp options in the 2 channel space than in the 5 channel space. I had hoped to get a class D 5x single chassis amp but but could not find a good balance of price and quality. That will probably improve within a year but I didn’t want to wait. For your needs, I’d lean towards the March Audio.
I have the Denon 4700 and the Monolith 7x200 plus an Adcom 5 channel. This combo allows me to setup a 7.2.4 configuration with the Denon 4700 in preamp mode. This setup sounds great! Multi-Channel mode can really open up the sound stage with the right speaker combination and placement. It's like being at a concert.
I purchased the March Audio P452 Purifi based amp and placed it on the fronts as I wanted to try the clean sound everyone raves about with the Purifi. Unfortunately, I found when I use Multi-channel or Auto-3D modes with the lower Purifi 26.5dB gain amp - it's a poor match when Monolith 7x200 drives the other channels. Even if Audyssey level matches the speakers initially, the levels don't stay in sync as volumes change. This difference in gain between the amps is a non-starter. The fronts on the Purifi sound really low at lower levels, it's almost as if they are 50% lower than the channels driven with the Monolith amp.
While the Purifi sounds good by itself on a 2 channel setup, I can't recommend it with a multi-amp (with different gains) home theatre setup - the only exception would be if all your amps were Purifi - and that's an expensive option.