The acoustats will sound best with tubes that can deliver a good amount of current. That said I’d also pick an EL34 amp over a KT88 amp for electrostats given the sonic signature and midrange magic. Unfortunately EL34 are lower powered, so if you have a larger room you will benefit from EL34 monoblocks. Or an integrated or separate amp using at 4-8 EL34 tubes (4 should be fine for a small/mid room, 8 will be needed for a larger room).

I’ve heard the Pass 150 and I wouldn’t pair it with the acoustats as it has a less warm, and thinner character compared to much of Pass’ other offerings. 

Forget about any amp less than 200wpc. With a sensitivity of around 81db/meter the 1+1's need a lot of watts to come alive. My Perreaux 2150B with its 920wpc@2ohms would be a good match. Another good amp would be the Acoustat TNT - designed by Jim Strickland to work well with his speakers.

It depends on you room size. Just yesterday I went over to my buddy's place. My buddy had my LSA wasp class D ...125 wpm into 8 ohm amp at his place for 2 weeks. . He has the acoustat, not sure of model, about 7 feet tall and maybe 12 to 15 inches wide. His room is approximately 17 ft by 21 feet. He had been using NAT mono blocks, but couldn't get them to drive the acoustats. His jaw dropped when he heard this amp with his speakers. he subsequently, spent many hours in the music room. I can only guess that the current the amp produced is what drove them satisfactorily.