I hate to say it but the idea of using dual M80's for the center channel got to me.
I will take the doors off the cabinet just to try it out.
Only harm will be if I waste a couple hours swapping them back and forth.
Two day weekend coming up in 3 weeks.
I think this is an arrangement that many of us(space and pocketbook notwithstanding) would love to have. If you have either one or in your case, two M80's as center channels along with L/R M80s, you will get that perfect timbre match in the front sound stage that is just not possible with any horizontal center. Also the vertical speakers will give you a truer wider angle of dispersion so toed-in in such a manner to hit all the angles it should sound just fine.