Lee, there shouldn't be a good reason to worry about "possible distortion" if it isn't actually occurring. Your present Denon should be able to cleanly handle brief peaks on the order of 103dB with your M80s. The question as to whether they're being called upon for more than this(disregarding the low bass frequencies handled by the sub)should be determined by measurements with an SPL meter done with the sub disconnected, but leaving the crossover in effect so as to continue to relieve the M80s of the lowest bass burden.
The increased maximum power levels which you're considering wouldn't have any effect on "detail", but would simply allow playing brief peaks at a level about 3dB higher(e.g., 106dB rather than 103dB)with low amplifier distortion. Speaker distortion itself would of course be slightly higher.
Upgraded features may be good reasons to consider a receiver such as the X4000 with Audyssey XT32, but the increased maximum power capacity alone isn't likely to justify the purchase.
