More power for moderate listening levels?

Can you be more specific when you say you hear strain or lack of clarity at higher volume levels — maybe give some specific examples?

I agree that auditioning is the only way to go to see if one amp is better sounding than the other. Just curious about the effect of power at lower volumes.

At high volume I think the amp is staining because horns and cymbals are becoming harsh and not as musical. Overall, music is just not as pleasant and I gather it's not just due to high volume irritating my ears.

Appreciate the thoughts y'all.

How loud are you listening?  If you're trying to do 110dB, then there are better loudspeakers than the Harbeths.  Also, you mention that you're further away from the loudspeaker and dancing (presumably standing), both of which will alter the tonal balance of the loudspeaker.  Do the Harbeths sound strained at your normal listening position at high volume levels?

What is the volume level setting when you crank it up and notice the strain?  The XA-25 (same amp used in the INT-25) was tested by Stereophile at 80Wpc into 8 ohms and 130Wpc into 4 ohms. It does switch to class A/B at higher power output levels, so perhaps you are hearing that change?
I'm using the Pass INT-25 with Sonus Faber Electa Amator III speakers, 87db, 4 ohm, probably a more difficult speaker to drive than your Harbeths.  With my chair at about 8 feet from the speakers I usually set the volume in the range of 40 to 50 depending on the CD (63 is the max volume). I've also wondered if a higher power amp might drive these speakers better, although I don't think I'd want to give up the INT-25 for moderate listening levels.

There is nothing inherent in the design of an amp which will mean a higher powered amp will sound less good at low volumes than a low powered amp. The Pass Labs Int 250 may sound better or worse at low volume to your ears through your Harbeths than the 25. Whichever, it won’t be because of the amp wattage that there is a difference.  Whereas at high volume the amp output obviously is a defining factor.