L2, welcome. The setting on the receiver's volume control doesn't directly translate into volume level and it would be more informative if you had a sound presssure level meter that would give you a reading of the maximum sound levels that you were actually hitting. That being said, it would be unusual with most program material for an amplifier to be driven into clipping at a volume control setting of only -15dB unless it's developed a defect. Was this present with your previous speakers?
Assuming that there's in fact some problem with your Sony and/or you just want a receiver more modern features, there's no need to have to consider either the 8ohm rating of the M3s or their 175 watt maximum power rating. 8ohms in itself never causes a problem and you're certainly not going to get a unit that exceeds 175 watts(or would use anywhere near that power even if you did). Your M3s are about average in sensitivity and use about 1 watt at a comfortably loud level. It's also unlikely that even on brief peaks that usage would reach 100 watts.
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