Lee, this may sound simplistic, but the first thing is to check to make sure that both tweeters are actually working. Verify that the connecting strips on the terminals are tight and listen to a high pitched tone with your ear near the tweeter.
A basic way to increase upper mids and treble is to try a 3-4dB boost on the treble tone control to see if this improves the openness at low to moderate volume.
Yes, a new receiver may help, but it has nothing to do with needing more power; at the levels that you use, your current receivers are fine with respect to maximum power. This would be even more the case with the M80s, which use a bit less rather than a "bit more" power than the M60s. I'd suggest that the benefit of a new Denon or Onkyo with Audyssey MultEQ would lie particularly in Dynamic EQ providing a better tonal balance(with the "Flat" curve)at lower listening levels.
Yes, the Steinberg is another of The Planets that I own.
