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

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).

I would not recommend extremely unbalanced allocation of funds for any given type of component because the results will not be optimal.  But, that is not to say that there is a limitation on the ability to hear an  improvement in a source component or amplifiers because of limitations is a speaker as good as the 30.1s; they are plenty good for hearing the qualities of any amplifier.  I've heard impressive demonstrations of $50,000 amplifiers playing very modest speakers; differences between the amps being compared were very easy to hear even if the speakers were "modest."  Sometimes one does not know how good a particular speaker is until it is played with an amp that it is not commonly paired with because they components are supposedly not in the same class.

I heard a comparison of two EL34 tube amps that happen to use the 30.2 speakers in the comparison.  It was fairly easy to hear the differences between the two amps that were similar in design.  That is what one would expect with any decent pair of speakers--differences in other components would be revealed by the speaker.  I don't see why anyone would want to claim that their speakers are so crude as to obscure any differences in upstream components.