Hello,
If your interest is the ARC Ref. 110 or Ref. 210, either would be fabulous. The 210's though are mono blocks and have twice the power of the 110's, so that problem or concern is already taken care of. I would only go with two Ref. 110's though as they are stereo amps and could be vertically biamped for mono block style operation and higher total power since two are used versus just one.
OTL's like Atma-Sphere, that are output protected as is highly important for OTL's, (try to stay away from Tenor and Joule Electra in that aspect), are fine with the higher nominal 8 Ohm impedances, which I think (I may be wrong) both Usher models have. However, having listened to several Atma-Sphere models on various speakers, the more power that you have to each speaker the better ... just like any other kind of amp; be it tube or SS. Ralph's Atma-Sphere's sound even better with the power supply upgrades of various sorts. Try the 140watt model mono blocks perhaps.
These two speakers like (react very favorably to) amps (tube or SS) that have sheer kahunas under the hood in terms of watts or current delivery or both.
SS amps like those you mentioned (by brand at least) would be excellent SS power amps. as they have what I mentioned above: guts and then some to drive and control the speakers very, very easily depending on the specific model. I have found that the new Bryston SST^2 series are phenomenally powerful and have tons in reserve as well as a non existent noise floor if your AC mains are up to snuff and conditioned properly. They stay out of the way and let the music flow through so transparently and revealingly without sounding clinical like an old Krell.
Ironically, one of the best set ups I heard with either of these speakers was with Parasound Halo gear. The new JC-2 preamp and the JC-1 mono blocks were fabulous and then some. They can be had for about half price right here on Audiogon for about $3,500 or so per pair usually and are true 25W class A too.
I would strongly suggest to stay away from Cary Audio's tube amps. as they are syrupy, sluggish, overly warm, highly colored to sound euphonic intentionally, etc. I know... I worked at Cary in 2004 and heard them all. They would be like having great pancakes with far too much syrup on them. They seriously veil the openness of these speakers especially with the Be midrange too. That makes a gorgeous sounding difference over any other brand or model of speakers that I have ever heard. ARC, Atma-Sphere, or Bryston or Parasound Halo.