...there is a bit of hardening to the sound at 12:00.
Thanks for the additional info, Mjcmt.
Roughly speaking, a typical single-turn rotary volume control such as the one on your Mystere will provide around 22 db or so of attenuation at the 12 o’clock position. Your Yamaha Blu-Ray/CD/SACD player has a maximum output that is nominally spec’d at 2 volts. The approximately 8 db of gain provided by the amp at the 12 o’clock setting (30 db max gain - 22 db of attenuation by the volume control) will boost a 2 volt input to about a 5 volt output.
5 volts into what I suspect is the 4 ohm impedance of your speakers corresponds at most to 6.25 watts, and to less than that to the extent the speaker’s impedance is not purely resistive.
So as I see it you are not turning the volume control up high enough to utilize most of the power capability of the amp. However I have no thoughts as to why the sound is hardening at higher settings of the control.
Regards,
-- Al