Power amp for Modwright LS100

All good. It's going to come down to what you like. Everything noted on your shortlist is very nice stuff. I own a LS36.5 so I'd say start with ModWright and Pass if you're staying SS. If you're thinking tubes then I think it gets more difficult but can pay off. If you're thinking a tube amp, don't overlook maintenance, heat and tube replacement. If you're leaning towards SS, with your speakers I'd maybe move ARC and Bryston to the bottom of the list. Personally not a big fan of current ARC SS amps and not sure I'd go with Bryston, although very nice amps, with your speakers. I'd look at at one of the lowered power Pass XA amps - XA30.8  to start with or a ARC Ref 75SE; if new is a little outside your budget they are often on the used market. As much as I love my Pass, I'd say look into the Ref 75 with your speakers. Good luck.

I have a Pass X250 with my ModWright LS36.5. My experience with a big SS amps with a tube preamp it's best to run balanced cables. My previous ModWright SWL 9.0SE was a little noisy too which is why I sold it and got the LS36.5. Your formerr LS100 has an XLR out, but it's just a cheater - not balanced.  Pass X250.8 with the Ayre K1XE should be very nice. Run balance cables if not.

I wouldn't think the LS100 would be noisy. Larger Pass amps can get a little noisy because of their high gain running single ended cables, so balanced is highly recommended on them. Both Pass and ModWright make great equipment and as always it's all about the synergy between any two components. I spoke to Dan before buying my LS36.5, and he stated on my Pass the LS100 would not be any quieter than my former SWL 9.0SE because the XLR Jack on the back of the LS100 is only a jack on a single ended output. Abucktwoeighty mentioned his LS100 was noisy on his Pass X250.8 which doesn't surprise me. As far as his misfortune of tubes goes, that probably comes down to not buying from a reliable source maybe?

Back to Bluejeans on the amp search. What is your budget? I don't believe your Harbeths are a difficult load, so I would recommend quality of power over buying a bigger cheaper amp.