Harbeth 30.1 or 40.1

@arturgorniak is the extreme case of someone who believes 40 series Harbeths need serious power.
https://forum.audiogon.com/discussions/harbeth-40-2-40th-anniversary-with-simaudio-mono-blocks
At the other extreme, I've read posts from people driving them with tube amps well under 50 W.
But I think you are reporting the lowest.
For a real comparison/contrast, I think you need to aim for 100 or 150W SS.

I heard my friend's 40.2s with the 48W Line Magnetic LM805IA. Sublime.

@twoleftears I downloaded an app and finally got the number on how loud I listen to.  My max volume is around 90dB.  Average can get up to 85dB.  There is also a weighted average which I don’t really know what’s the difference, but it is much lower at 78dB.  I am at about 3.5-4 m away.  
I got a suggestion from someone that, I should leave about 20dB head room, ie. 100dB at 3.5m. Then I plug into a calculator in a web site, it turns out the following:

60W -> 94.95 dB
80W -> 96.2 dB
100W -> 97.17 dB
120W -> 97.97 dB
150W -> 98.93 dB

So, it seems your suggestion of 100-150W is spot on.  Then, it seems the only option is SS.  I am a tube guy, have switch to tube amps 8 years ago and never look back.  I am a bit hesitant to go back to SS gear.  So, my first decision need to be made is, do I get 60-80W tube amp, or 100-150W SS amp.

i read comments both ways.  Some say he try SS and don’t like it, then get a McIntosh 275 and very happy.  Some say he move to Luxman and Accuphase and they are as good as tube.

@gte357s   Yes, I agree it's tremendously confusing.

There have been reports of really excellent performance with Vinnie Rossi products, that aren't that powerful and have a tube front end.

Likewise, many swear by the combination of the LFD NCSE which is only 70W in its most powerful incarnation.

big_greg just posted above, and I also know of someone very happily driving them with a Linear Tube Audio LTA Z40 integrated, which is under 50W.

So there's a real diversity of opinion; I can't think of an example where there's more diversity, actually.

If you like the sound you're getting, perhaps there's no need to change.

My suggestion was to perhaps try to borrow a powerful SS amp just to see what difference it made (particular in the bass region).  On the other hand, tube watts do tend to be more powerful than ss watts, so perhaps try and borrow a 75W push-pull or ultralinear tube amp.

If you go on the Harbeth User Group website, you'll find a thread with owners posting lists of amps they've used.  Shaw has some pretty extreme views on amps, but it's very enlightening to see the range of models people are listing.

If you choose to go with a solid state amp, the Plinius SA 102 gave me the most satisfaction of the solid state amps I tried with my 40.1's.  There's one recently listed (Listing ID: lisa4409) at a reasonable price that was recently recapped.  I still prefer tubes with my 40.1's (Music Reference RM9 MK2, McIntosh MC275 MKV, VAC 70/70)... but the Plinius was still good... just not a s good as the aforementioned tube amps.  If you are a tube guy as you say, you won't be satisfied with SS,IMO.