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

Suncoast audio has a new YouTube video up of the M30s being driven by Luxman’s new limited edition Integrated, and it sounds fabulous.  I understand the new integrated amp is similar to the 590.  Luxman and Harbeth are a great match.  I have the M40s with Luxman separates and love the combo.  But I’ve never heard Herron, so I couldn’t say if it would be an upgrade.

+1 for a new cartidge.  An Audio Technica AT ART9 XI for example.  

Had one. Hated it. Exactly the opposite of the sound I enjoy but that’s just my ears.

I know you are asking about amps, but IMO, contrary to Alan Shaw's opinion, speaker cables make a big difference.  I've owned the M30s, SHL5s and P3s and I've found the right speaker cables to improve the sonics significantly.  I don't know what you're using now, but if you have to move the speakers often, I would see if the Cable Company has any more Audience AU24 show demos at 1/2 price.  Excellent sound and very easy to work with.  I also love Purist, but they are not the cables to be moving around on a regular basis.  Not that you can't, but they are kind of bulky compared to Audience. 

I would see if the Cable Company has any more Audience AU24 show demos at 1/2 price. Excellent sound and very easy to work with.

FWIW I have used AU24E with my 30.1 all along, and have been quite happy.