Doozy of an amp question

I'd guess the Mac would be easier to resell if you ever wanted to swap it.

You didn't specify a budget but my forever amps would be the pair of Nagra HD mono's listed on the 'Gon if I could afford them!

I can’t answer your question directly and admittedly have not heard the Macintosh amp. I have heard  the simaudio. I’ve also owned a couple of  SimAudio products and I found them to be very well built, reliable, and competent.

I can appreciate the difficulty you have with in-home auditions. Even if you can’t do that, I suggest you try to listen to these components at a dealer location because I think the McIntosh house sound and the sim  house sound are quite different. it seems likely if you like one, you may not favor the other. The Macintosh amps I have heard tend to be sweet, critics would say dark and veiled, proponents would say musical. The simaudio Amp is more detailed and transparent, critics would say dry and analytical although that would not be my view, I think it delivers to the speaker the signal it receives without coloring it inany way, not a bad attribute for an amp. Although it’s always better to listen in your own set up, I think if you could hear these two components at dealer locations, you could at least get a good read on which House sound you like because they are very different.
good luck!

I would look for an ARC amp to match the preamp you own

There's no way of knowing in advance how two components will align with each other. Actually, three components, the speakers, power amp, and preamp. I'm not an ARC guy, but I agree with jperry's comments that ARC is the safe choice. Anything else, the results could be anything, and odds for a mismatch are good.