Opinion re/Parasound A23 with Vandys?

That will probably be more than plenty, except perhaps for very large rooms.

I usually use a 150W amp (a BAT VK-250 or a Bryston 3B-SST) with my Quatros and have never come anywhere near the limit of that much power.

While waiting on some new tube monoblocks, I've been using a 60W Bryston B-60, and even that has enough power for me, though if I wanted to listen at insane volumes it might run out of gas on Mahler symphonies.

My room is an irregularly shaped open plan living/dining room with vaulted ceilings -- about 400 square feet in area and 6000 cubic ft in volume -- and I sit about 8.5 ft from each speaker.

Well, I am not sure if my input will help as it is not with the Quarto or Model 5, but with the 3A sig. the A23 was not acceptable. I even tried using 2 A23's as mono block in a couple configurations and it sounded better, but far from good. The sound was closed in and dark. I think it was loud enough, but just sounded poor. I upgraded to an A21 and the change was drastically better and I am still using it a couple of years later. I really enjoy my Vandersteen and Parasound A21 combo.