More power for moderate listening levels?

1- Do you like the speakers ?
2-Don't listen to the B.S
3-You need more power for those higher listening levels, Get it !
4-Lot's out there, volumes of B.S. out there as well, you'll just have to wade through.

That Pass is a good amp but only 25 w/ch.

WHat you are hearing is most likely the amp clipping, ie struggling to deliver the power needed at higher volume to those not very efficient but very good sounding Harbeth speakers (if driven to the max properly)..

You need more power to go louder and clearer. It’s that simple. Basic physics and hifi 101.

Along with more power, more current delivery capability can again only help. From what I read, that is likely the case with those Harbeth. You need a better quality amp with higher quality power supply for that.

Higher damping factor specs and ability to double power into 4 ohms is a good indicator of a beefy amp capable of delivering lots of current when needed.

That is what you need to play it safe.

What amp specifically? Harder to say. A good quality Class D amp 200 w/ch into 8ohm, 400 into 4 or more would be a good bet. 100 w/ch to 8 ohm, 200 to 4 might even cut it.

It all depends how loud you need to go. Clipping is public enemy # 1. YOu want to avoid that at all costs. Power and current is your insurance policy for that. 25w/ch won’t cut it with most speakers out there for higher SPLs that are loud and clear.

TUbe amps are popular choices with Harbeth and can sound very good, but a higher power tube amp capable of driving Harbeth to their max at higher volume could cost some serious $$$$s and will consume a lot of power whenever on, whereas Class D is a reasonable alternative that should lower cost and keep power bills down.

Most tube amps and Class D amps soft rather than hard clip, so that will make it seem like fewer watts can go furtherbut will limit absolute SPLs and dynamic range. Still more digestible kind of distortion  to the ear than hard clipping. ANother reason tube amps and Class D are both reasonable choices depending.

As long as you can avoid clipping as described, specific amp choice will matter less. They all will be up to the task. Can’t sound good if not up to the task......not as hard if so.

If you enjoy the Pass sound I would certainly step up to a 250.8 you will still be getting the first 18 watts in class A where most of your listening is and plenty of reserve for loud listening. I have one powering my Maggie 3.7 and never hear any straining.

It really comes down to priorities and preferences. If high quality at high volume is a priority then you need different speakers. Its that simple. The power demands of those speakers at high volume, forget reserve, you need hundreds of watts. Need. To use. Not have in reserve. Whatever that even means. 

Tekton makes some great speakers, very high efficiency, great sound, and their favorite amp is Pass. It would seem a match made in heaven.  

But if on the other hand you're totally in love with the medium volume sound, able to understand what's going on and live with the compromise at higher levels, that's fine too. 

Most of what I see here though sounds like trying to have it both ways. Since horses for courses didn't gain any traction how about don't put legs on a snake? You have some fine speakers, for what they are. Don't try and make them what they're not. Doing something up a rope, or into the wind, comes to mind.

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.