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

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

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?

@honeyooi been down the cartridge route. Spent thousands and always prefer the tone of the 103R, although now I use an Anamighty-built one that costs quite a bit. I have never found a cartridge that duplicates that tone…. with the right SUT of course.

Your fatboy arm deserves better cartridge,
Me too have denon from Zu Audio & both upgrade stylus and still in my system as back up!
but once you move up the chain to 2k cart you hear more and more from record that you never hear before ,everything just more open and clear. So upgrade your cartridge first .
im also use harbeth M30.1 and stuck too because room size .

For my musical taste (acoustic jazz, guitar, classical and jazz piano, cello, guitar based acoustic blues, that sort of stuff) in my space limited co-op on Long Island I'm not sure what's next.  I'm 90% digital/streaming with Qobuz and don't want to upgrade the turntable for now.  I've been thinking speakers but based on what I know after 30 years in this hobby (and with a budget of $5 or $6k) I don't think there is "better" than the M30.1's.  

My system

• Pass INT-25 integrated amp (recent)
• Harbeth M30.1 
• Innuos Zen Mk3 streamer/server (recent)
• MHDT Orchid DAC
• VPI Scout / Dynavector 20x2L cartridge
• Sutherland 20/20 phono stage (recent)
• Mid range interconnects
• Triode Wire speaker cable (recent)

I'm not really itching to upgrade anything. An audiophile conundrum.  

Put the turntable on Townshend Pods (or Platform) it will be a huge upgrade. Put the speakers on Podiums, if you can. Might not be convenient if they are moved around a lot. Synergistic PHT and ECT on the turntable, HFT on the speakers (and walls, if possible) and ECT on the amp and other electronics.  

Do all these the improvement in detail, dynamics, truth of timbre and imaging will have your jaw dropping in amazement. You will forget any thoughts of "diminished returns" and be trying to figure out what else you can do. When that time comes PM and I will hook you up.

Maybe a new cartridge...

Thanks and sorry, I should have mentioned that I’m stuck with the 30.1s due to space limitations that require me to move the speakers in and out each time I listen. I will buy the 40s in a heartbeat if we ever move!

An easier upgrade route might be to sell the M30s and buy the M40s.

Few would argue it's not a clear upgrade.