I think the reason it is hard to get an unambiguous answer is the dynamics are not strictly one component and different systems react differently. When I had solid state amps more power made a difference in impact… making them sound better at all volumes but they still sounded best in the high 70’s to 80’s.
If you like the character of the sound then I would not recommend switching to a solid state amp. The character will change too much. That is a really nice amp.
The volume at which my systems have sounded good over the last fifty years have continually dropped as my system has gotten better. My system now sounds the best it has at low levels (see my system).
If you were going to do one thing, I would definitely trade in and apply the $10K to new speakers. This will put you in a whole different category of speaker regardless of the actual choice. Secondly,I would recommend looking at dynamic… like Sonus Faber, or DyneAudio. These will give you solid bass and natural presentation at all volume. I have had various electrostatic and ribbon speakers over my life and while they haves strengths, they have many weaknesses. All my planar speakers required pretty high listening volumes.