I went to a small UK show, run by one company 2 weeks ago and it was one of the best I have been to. It even had Dan D'Agostini talking about his momentum monoblocks.
To get to the point, 2 of the 4 rooms had ARC amps, both the reference 150 and 75, on different speakers, but next door to each other. The first room had Wilson Sophias I think, initially with a Krell amp, ouch. Harsh, edgy sound. Then a reference 150 was put in instead, a collective Aaah..., from the audience. Just a lovely detailed but smooth sound, big soundstage. No loss of detail compared with the Krell, but a really musical, not etched "HiFi" sound.
Moving on to the Ref 75's, with Vertere speakers I think, a very similar sound, all the charectaristics I like. The store owner, Trevor, who I know quite well, said he has a number of customers changing from Ref 110 to Ref 75's, if that helps with the question of Ref 150 against ref110's.
My impression, for what it is worth, if your speakers are sensitive enough, the Ref 75's may be the sweetspot in the range. A review by a long time ARC fan in HiFi News a UK mag, suggested just that. He opined that for Tube amps, an output of about 75watts was optimal, with so many great amps, including ARC ones, at that level. I am sorely tempted by the 75's, when I can get the cash together.
Finally the room with the Momentum amps was in a different league, with larger Wilson speakers, I can't forget the model I'm afraid. With the new Momentum Pre, I don't think I have heard a better sound, ever. Mind you they are in a different price league too.