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

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.

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

He discourages it to the extent the forum was split into two sections, one for technical discussion based on facts and science while the other section was for subjective discussion on amplifiers or all things audio not endorsed by Harbeth. Your contribution may be welcomed at first, but after a while they will be shot down by the moderator and the cohorts if they don’t coincide with the values that the clan preaches. As Whipsaw had suggested above, your opinion or post will mostly be disparaged.

If you go a step further, the post which you made will be modified by the moderators or completely removed. I have forgotten if some users have been banned for posting few issues they experience with their new speakers. Issues such as glue marks on the main driver and defects on cabinet etc.

The forum used to be a free view as a non-member can read the content of the forum. It now requires a member to log on to the forum just to enter the forum. You wouldn’t be able to gain access to the content on the Harbeth site before you become a member.

I have forgotten if some users have been banned for posting some issues they experience with their new speakers. Issues such as glue marks on the main driver and defects on cabinet etc. 

This is an interesting issue when you think about it. It’s tough to conceptualize a business creating a public forum that includes people complaining about their products. I would think somewhere in the forum rules it probably states that matters such as those should be handled directly between the customer and the company. Or at least I’d understand if those were the defacto rules.