Change from PS Audio S300 Stereo Power Amp to PS Audio M700 Mono Amps

I have had the S300 Stereo Amp for about two years driving Vandersteen Treo floor standing speakers and have been very happy with the overall sound quality and never disappointed but have had the itch for a change so recently picked up PS Audio's M700 Mono Amps.

After about thirty minutes of listening on the M700's at a moderate level of about 60 dB I start to feel pressure in my ears. Over the past three days stopping and starting at different times of the day I get the same ear pressure after a short time listening. To note everything else in the system is exactly the same and the new amps are in the same position as the old amp. I am going to give the new amps more time but am thinking that the M700's are possibly not a good match for the Treo's where the S300 was a very good match.

Does anyone have a similar experience trying a new amplifier or have suggestions as to why this may be happening?