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.

Back in the day there was an Acoustat dealer that I visited on several occasions. He drove the speakers with the VAC Renaissance 70/70 amplifier (Push-pull 300b, 65-70 per channel) with truly excellent results. Granted these amplifiers were built with very stout power supplies ​​​​​​.

I did not realize that their sensitivity was that low (81db) so I understand The recommendation for higher power solid-state amplifier. But, I can attest to the fact that they were marvelous sounding driven by the VAC amplifiers. The speaker probably has a friendly impedance load.

Charles

Sorry, forgot to mention, he also had a pair of subwoofers along side of his acoustats. I must say I was very impressed with the sound I heard, considering the age of the speakers. He also had mentioned that he thought they were 82 or 86db efficient .