Which solid state power house: Naim NAP 500DR or McIntosh MC611?

There’s no such thing as all else equal. The amp highly depends on the speakers it drives as well as the rest of your gears (your preamp, front end source components, cables, etc). What are your speakers and the rest of your setup gears? Your preamp, front end source components? It's all about system synergy and all. It's very important that your speakers have great synergy with your electronics and cables in order to achieve great end results. 

In general, the Naim NAP 500DR is superior to the Mac MC611, better musicality, musical rhythm, rhythmic drive, more refined overall. The Naim NAP 500DR comes in two box configuration with Naim 500 power supply unit. The NAP 500DR is a dual monoblock amp with each channel independent from each other and can be bridged. The amp is stable all the way down to 2 ohms.

The Mac MC611 is pretty solid monoblock amps with lots of power (600 watts @ 8, 6, 4, 2 ohms). Again it really depends on what speakers you have and the rest of your setup gears.
The Naim NAP 500DR retails for $22k whereas the Mac MC611 retail for $14k/pair. If you have $22k to spend, go with the Naim NAP 500DR. But again, it’s hard to make recommendations not knowing what your speakers are and the rest of your equipments eg your preamp and front end source components.

@jeredh 

$75k for just the power amp or for everything? 

Well jeredh just to let you know that the Naim 500DR power amp alone is $22k, and if you go with the Focal Sopra 3, which retail for $19k/pair. The power amp and speakers will cost you $41k.

I highly recommend that you partner the Naim gears with Focal Sopra 3 speakers. The Focal Sopra 3 were actually voiced with Naim 500 DR amp + Naim NAC 552 preamp since the merger of the two companies. They will sound great together and have great synergy. System synergy is very important in high end audio or hifi. Fyi, Focal acquired Naim couple yrs ago.

Naim also makes great analog linestage preamp (500 series) to go with the Naim 500DR power amp. The Naim NAC 552 linestage preamp will run you another $22,400 including its partnering external power supply unit. Now so far you are at $63,400.

And for digital front end the ideal partner for Naim 500 series gears would be the ND 555 streamer/DAC and I suggest you add the 555 PS DR power supply unit to the ND 555 to significantly improve sound quality of the ND 555. The ND 555 streamer/DAC costs another $19k plus the 555 PS DR power supply unit for another $9k to $10k.

So, the total costs will far exceed your $75k budget. These do not include cables. But you will be more than satisfied with the overall performance with these Focal Sopra 3/Naim 500 series gears combo. The Focal Sopra will have great synergy with these Naim 500 series gears. They will sound spectacular together.

It will cost you much less if you go with McIntosh MC611 amps and the C2600 preamp/DAC since the MC611 monoblock amps retail for $14k/pair and the C2600 is around $7k. That’s it. For electronics alone they will run you for only $21k plus speakers but the overall musicality and sound quality won’t be as good as the Naim 500 series separates (NAP 500DR amp w/ it’s external PSU, NAC 552 analog preamp w/ its external PSU, ND 555 + 555 PS DR PSU). Not even close imo.
And the Focal Sopra 3 will sound better paired with the Naim 500 series gears.

Btw, as far as speaker choices, you shouldn't ruled out Wilson Audio and Sonus Faber. They both make fantastic sounding speakers. I personally prefer them over the Focal. But again when it comes to speakers choices it all comes down to personal taste and preference as well as your own musical tastes. 

@jeredh 

I've heard the Wilson Sasha series 2 paired with all Audio Research Reference series gears in a very well treated room and thought they sounded fantastic together. I've also heard the Wilson Alexx, which is way better than the Sasha 2 and they should be considering the price difference between the two, paired with 2 pairs of Dan D'Agostino Momentum M400 monoblock amps + D'Agostino Momentum linestage preamp + DCS Vivaldi full four stacks served as digital front end source components and they performed fantastic together. The listening room was very well treated. The Wilson Audio speakers usually pair really well with D'Agostino amps as well as Audio Research. I've never heard the Wilson paired with McIntosh gears before so can't comment.

Sonus Faber seem to pair really well with both McIntosh and Audio Research gears. I have heard various different models of Sonus Faber speakers paired with either Audio Research and McIntosh amps before and they sounded great together. They also pair well with some Soulution & CH Precision gears. 

@jeredh 

What did you end up getting? 

@jeredh 

Great choice. Are you using the Naim ND 555 streamer/DAC with a 555 PS DR power supply unit? You should have gotten the NAC 552 linestage preamp with a 555 PS DR, not the n272. It will make huge difference sonically. 
Additionally, you might want to add a Naim Uniti Core Reference music server to connect to your Naim streamer/DAC. It will make a big difference too sonically. It will sound better than having your computer or NAS feeding your Naim streamer/DAC. 

I am a big Naim fan boy myself. I personally own the Naim Statement NAC S1 linestage analog preamp and the Naim Statement NAP S1 monoblock amps driving a pair of Magico M6 speakers in one of my reference setup/system. But I'm not using a Naim digital streamer/DAC. I have a Clearaudio Statement v2 turntable with a TT-1 Statement tonearm, Statement Goldfinger cartridge, external separate class A linear PSU for the turntable & a D'Agostino Momentum phonostage pre. 
For digital front end source components I have a MSB Select ll DAC with its flagship Femto 33 clock and 2 mono separate powerhouses or 2 mono power supply units (one feeding power to its digital side of it and the other feeding power to its analog stage), MSB Select transport with its external separate PSU, Naim Uniti Core music server. 

I also have a Naim Uniti Nova all-in-one box integrated solution (streamer/DAC/integrated amp) driving a pair of Sonus Faber Olympica l standmount speakers in my bedroom setup.