More power for moderate listening levels?

Cheapest solution is to bi-amp using 200+ wpc of class D for the bass
speaker.
Use the Pass 25 on the upper only. 

Also borrow/buy someone's measuring equipment to see if and when the clipping occurs.

Then report back.

If you want to go louder and clearer you can change amps and go for more muscle or change speakers to something that is better suited to 25 watts.  But they won't be Harbeths.

I found myself in a similar situation. I drive Sonus Faber Sonettos II with Pathos Classic One MKIII. Although I'm very pleased how it sounds at moderate to loud listening levels, every time I crank the volume up I feel like I miss dynamics and grunt from the amplifier. It seems the amplifier is having a hard time driving the speakers when reaching 85db+ SPL at my listening spot which is no more than 9 feet from the speakers. 
I'm on the same crossway as you. I really like the sound of the pathos but I always feel I need more power for the speakers to shine ...

Tekton makes some great speakers

IF one likes that sort of thing.

In the Monitor review, Stereophile determined "My estimate of the Impact's sensitivity was 87.5dB(B)/2.83V/m, considerably lower than the specified 94dB." https://www.stereophile.com/content/tekton-design-impact-monitor-loudspeaker-measurements

Truth in advertising: Miller owns Tektons.

If I measured them they would be different again. As I said, I've seen a LOT of fudging and manipulation, of the numbers. How that mic is held what it is held with, background noise, I've seen some weird stuff to get NUMBERS. Wiring out of phase is real big.  Listen to the difference, when the mids are NOT wired out of phase... and when just one of three (EX; mid drivers) is wired out of phase, the number look good but  they sound "FLAT" because they are, they measure flat...

Tekton does not wire out of phase.... I don't know Mr. Pass views on  crossovers, but I do have one of his hand built First Watt  OXO.  Nothing more than 18 db XO, more like 6 and 12 db.. His amps are going to reflect sounding better, with a lower order XO, too.. Those long slopes help a lot when it comes to power demand...and lower watts.

24, 48 db slopes, and any low watt amp, the amp will labor, AND run out of gas.  Even if the E factor is high.... 25 watts is 25 watts..I have to look up the specs on the speakers...I have a hunch about the crossover...I just wonder if it is in the specs without, looking at the actual crossover.

Regards..