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

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.

Alan Shaw has long welcomed any debate on his opinions regarding cables and amps. 

In fact he once even offered a pair of his top of the range M40s to anyone that would be willing to undergo an unsighted amplifier comparison test.

Level matched of course. 

Unsurprisingly, despite the endless online amplifier hullabaloo / advertising / promotions etc there were no takers.

There were no takers.

I would suggest the dear reader should let that sink in to their long term memory for easy recall the next time they come across some casually penned blowhard review extolling the unique quasi mystical properties of another routine amplifier.

Completely agree yet this discussion is not about ‘routine’ amplifiers. 

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.  

Alan Shaw has long welcomed any debate on his opinions regarding cables and amps.

This is nonsense. He largely discourages such debate, and considers his views to be facts, not opinions. And the condescension towards those who think otherwise is often palpable.

In fact he once even offered a pair of his top of the range M40s to anyone that would be willing to undergo an unsighted amplifier comparison test. 

Level matched of course.

Unsurprisingly, despite the endless online amplifier hullabaloo / advertising / promotions etc there were no takers.

I'd need to see the actual challenge, but the way that you present it is likely to be disingenuous at best. Shaw, and those who share his views, have been forced to admit that amplifiers can indeed sound different, but argue that it would only be true of those which produced a certain (unspecified) amount of distortion.

They scoff at the likes of Nelson Pass, who is fully transparent about his choice to "voice" at least some of his amps with a certain amount of Second order harmonics, because they are not as neutral as possible. 

Anyone who claims that all amplifiers sound the same is, to put it kindly, on very shaky ground.

One final point: level-matched AB-X testing is undoubtedly a very valuable tool, but in practical terms, very difficult to achieve. Two (or more) amps, using exactly the same speakers, interconnects, etc.?

Good luck with that.