More power for moderate listening levels?

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.

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.