Pairing MA 5300 with Large Advents for 900 sq ft apartment?

I would try the large Advents and the Mac speakers and see which sounds better in that room, you may be surprised.  The Mac amp has plenty of power for either pair of speakers.

I'm starting to think about downsizing even though it's a ways off. Pretty much decided to keep my large Advents and just upgrade the passive crossover. Keeping my Sony V-fet integrated, but not sure of the rest. Hopefully I will have everything digitized by then. If not, all my best source equipment will somehow get squeezed in.

This is a great question for MAC Labs and, as a MAC owner myself (actually, the MAC owns me), I would recommend you email this question to them. In my experience, they have always been very helpful with just about any questions you might have about their equipment.

That being said, with the MAC MA5300, I would strongly recommend you stick with 8 ohm nominal load speakers. A 6 ohm nominal load might be as pleasing, as long as it's relatively efficient or sensitive. However, all things being equal, I think 8 ohms is the way to go with that amp. Your amp can, of course, drive speakers with more resistance but won't get everything out of speakers like that, that they are designed to deliver.

much will depend on the building,  and how thin the walls and floors are, not simply which speakers you like best, but which will be more "apartment friendly"...I know, it's hard enough to pick a system without wondering about your neighbors...

If you will be using Roon, take the big ones. :)