Buy one of each. Then everyone is happy.
The -3db point on the M80s is 34Hz, for the EP175 its 32Hz and for the EP350 its 28Hz.
I have not found much difference between just the M80s and M80+ EP350 for a lot of the music I listen to. Where I do notice it is for Synth and pipe organ music where you have notes into the 20s.
I am not sure that the EP175 would add much noticable to your current setup. In fact, unless you listen to a lot of classical or electronic/pipe organ stuff, consider going without a sub as a possibility.
The M80s really are a capable speaker.
