Power amp for Modwright LS100

Have you considered the Modwright KWA100SE or KWA150SE?  Just an aside, if you haven't considered this for the LS100, might want to try Black Treasure CV181s (6SN7 equivalents) takes the sound to another level

Black treasure tubes are already installed. Bryston and Modwright amps are on my shorlist but hoping for some insights on amps from Rogue, Passlabs, AR and similar brands. 

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 presently drive Daedalus Argos V2 with Modwright LS100 & KWA100SE. Yes, the yare a different "animal" than the Harbeths. That said, they previously drove JM Reynaud Offrande Supreme monitors easily and musically.