Do the Boston Acoustics have dual 8s? (just curious). The dual 6.5"s put out a lot of bass, because there's a lot of surface area there, certainly more than a single 8. From my experience with M50s, I'd say that you won't need a sub. It would be nice, but almost everything's there with just the 50s. And the 60s and 80s should be able to put out more bass just based on the fact that the cabinets are larger.

I am the Doctor, and THIS... is my SPOON!