Would a stereo Sim Audio Moon 861 amp be as powerful as 2 McIntosh 611 monoblock amps?

For your 85 dB target level it won’t matter; either amp will be way way more than ample. It won’t matter till you’re pushing at least mid-90’s or above, continuous. Maybe not even then. 300W is a LOT of power. Most speakers are not rated to sustain that. 

You’ll almost certainly prefer the sound of one amp or another, but it won’t be due to lack of power capabilities. 

I was asking in this thread because I own a Yamaha AS 3200.  I played at 70% volume to get about 70 DB. After 70% it starts losing its sound quality really bad and sounds thin. I want to make sure my next smp has plenty of headroom

That really indicates something else is wrong. Not raw power. Your speakers should be pulling a fraction of a Watt to produce SPL's under 80 dB.

Volume controls are never linear, so 70% on the dial (or 70% of its max numeric value if digital volume) would be much less than 70% of full power. That said, if you have to go 70% to hit 70dB, then your system could be lacking the necessary gain to tap the amps' power reserves. Many modern systems are on the other side, having more gain than they need. A poorly matched cartidge & phono stage is the most likely culrprit for a gain-starved system.

Or you could be fine on gain, and you just don't like the Yamaha amp (or something is wrong with it), or the speakers, or the match of amp to speakers. If you're at volume 70 and the full range is 1-100, and each digit represents 1dB, then 70 is actually 30dB down from "full gain", which means 1/1000th (or 0.001 = 0.1%).