Are these 660s for your cabin? Still sitting about 8 feet away? The 660s might feel like you're sitting amongst the band.

I think the bass integration may be better in your room with 660s vs M5s. When I had my actives in my living room, which is about the same size as your space, the bass could not possibly have been any better!

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