Is there a point of diminished returns with amps and Harbeth 30.1s?

You're not going to get noticeably "better" by changing the amp; you might achieve something "different" (or it might sound remarkably similar).
If you have the urge to tinker, has the room had acoustic treatment?

agree with @twoleftears

to me the mon 30’s have a particular sound and tonal palette, those who love it will generally not prefer more tonally balanced and extended-range speakers that don't accentuate and emphasize the midband as much

good solid state amplification with some tubes earlier in the chain is the best way to get them to sound great

My suggestion save more money and get that LAMM ML1.1 .
with same manufacturer you got better synergy and full tube amp Lush .
but is hot though, But worth it .

I worship the ML1.1s but our 700 sq ft loft is just too small!

You can get meaningful improvement by getting a better amp regardless of the speakers you are using.  The question is whether the improvement is worth the price.  With speakers as good as the 30.1, I think that it would be worthwhile to spend a significant amount on an amp of your liking.  I know many Harbeth fans like high power, but, I’ve heard, and liked these speakers with 50 or so watts of tube power.  I think they sound good with less power than that, so I would look for quality, not necessarily quantity (I.e., watts).

I don’t know what $5,000 buys on the used market.  I particularly liked these speakers with the Synthesis A100 amp and their A40 amp (both tube amps).