I am fortunate enough to own an Audio Research Reference 160s stereo amplifier ($22K) and my dealer / friend has lent me a pair of Audio Research Reference 160m monoblocks for the last year. So I know them both well.
In the past (up until about two years ago) I have owned top of the line Pass amplifiers for nearly 40 years. I can tell you, both of the ARC amps simply leave the other amps in the dust. If all you want is detail and slam then the Pass x series is for you. But if you want natural, musical, and overwhelming powerful and real sounding amps… these ARC amps are simply incredible.
So, the difference. These are the same exact design… one in two chassis and one in a single. The moment I first heard them I noticed the monos had a wider and deeper soundstage and a bit quieter. My impression has not changed, but the degree to which the monoblocks are better continues to be bigger and bigger. Over time I realize how much better they are. Layer after layer i hear Draper and more emotionally into the sound. They are so musical. I occasionally swap them, favoring the monos, by a long ways.
So someone that doesn’t really know my system would be really impressed with either amp… but over time be more and more blown away by the monoblocks.
Since, our systems (at least mine) tend to be for a long time… most of my components stay for ten to twenty years. Absolutely worth the high quality amp… and the mono blocks.