Please know, I am not trying to be rude at all but I think you have asked a question that is rather backwards to be honest.
Most st people have speakers and try to ask about a matching amp or preamp.
You have done done something quite the opposite and it's difficult. It's like asking people what color car should I get or what is the best fish to cook, it depends almost entirely on what you prefer.
My recommendation is to try a few mac dealers and see what they have or try a major audio show.
As for your question on the 601's vs the 452. I can share this. Typically you can get better soundstaging with two amps versus a single amp and this is still the case with the 601's vs the 452. However for your size room I would suggest that without going deaf the 452 will be more than adequate. Further, the 452 is one of the most sought after units on the used market so if you want to change you will have no issues with the sale of the 452.
As as far as matching speakers, you would likely get as many opinions as possible. I personally love B&W and I think the new 803's might be near your range, but I also think the new design is ugly as sin, by they sound good. Revel might have a good match for you as well. My suggestion is that you need to listen to lots of speakers and decide what you like.
Realize that the McIntosh has a distinct sound when matched with speakers and often rolls the high end a bit.
I should think in many regards it might be easier for you to sample speakers and different stores and narrow you search to perhaps 3 lines and work from there. Hope this helps.