Your room sounds small. Even though it's open to the rest of the house, it will still get very loud with the M5s. I know because I put the M5s in my living room which is open to the rest of the house and drove them with my 17 year old Denon.

The low end from the M5s is very nice. It's not as deep as my old M80v2 but it's much better to listen to. So is the midrange. And the highs. And the soundstage. And the imaging. M5s over M80v2 any day! When you hear them, you'll hang yourself from the rafters for torturing your ear-brain system all these years. LOL!

You won't need that sub or maybe you can set the speakers as large and move the sub to a location that smoothens out the bass for the entire room. My Onk found the M5s go down to 40 Hz in my large space. I'd turn off my 800v4 and let my friends listen to music and movies and there were no complaints at all. Of course when I turned on the 800 they realized what they were missing...heh heh.

Regardless of which you decide on, foam plugs for the ports may be in your future since they'll be sitting close to the front wall. Axiom has custom ones that fit the speaker sphincters. Toeing in the speakers may also help with the bass. I've found because they are so dispersive, toe-in doesn't negatively impact imaging and may improve it depending on your room geometry.

Good luck with that room. I have the same problem in my living room with M3s (not the M3s' fault). Your Sony likely has some kind of room correction, which I don't have, so that will help.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated