...the biggest challenge I have seen with large speakers in small rooms is not being able to get them far enough away from the walls and getting stuck with too much bass reinforcement as a consequence.
That and forming an enjoyable soundstage.
As long as you set the M80s as "small" rather than "large" (so the M80s are not handling deeper bass anyways) I expect you will be fine, and may not even need the port plugs. You might have to adjust the crossover point up to 100 Hz or so.
This has not been my experience with my M80v2.
Sub placement is going to be interesting in a square-ish room - probably worth looking into that as well. My first thought would be "one right by the listening position (like an end table) and one near the mains" although having them both near the listening position might be fun.
I thought my room was "bad" or I didn't know how to place my EP600v2. But I realized over the years just how bad that sub was because when I brought other subs into the room, they sounded right; not nearly as deep or as powerful but tighter and faster.
When I first powered up my first EP800, I did absolutely no tuning and it sounded right. In fact, no matter where I moved it, it sounded right. In its final position and with tuning, it has cleaned up the sound of my entire system. No matter where I sit in my space, it sounds good. As I said in my review, the difference now between Audyssey engaged or not isn't as obvious as when the 600 was in the picture.
For Chaos, I advocate trading in the 500s and M80s if he has to for one 500v4.
