Murray, since the 990 has a direct input available to its amplifier section, the front channel pre-outs of a multi-channel receiver can be fed to them so as to use the 990 as an amplifier for the mains. Any difference in the maximum power ratings of the two units would be meaningless as to matching front and surround levels. In the calibration process, either manual or automatic in the HT receiver, the voltage outputs to the respective speakers would be adjusted so as to balance the levels at the listening position.
In my view, however, you're unnecessarily complicating things and duplicating expenses with two receivers. I understand that you don't have the 990 yet, and if this is the case, my suggestion would be to get one of the excellent HT receivers, particularly a Denon or Onkyo model with Audyssey MultEQ, now. Any of these with maximum output ratings anywhere in the 100-150 watt range would be fine for your M80s now and you'd be set for your future speakers as well. The Audyssey room correction would be available immediately even while you operate only the front M80s.
