Just spent the morning comparing the Pass to the TL12 Plus via the 149s. It really is fascinating, no clear winner, more a character thing.
The little Leaks are just so crisp and alive, they sound huge, are really happy at low volumes and I just instantly relax with them. There is something just so ‘right’ about the way they do things. They make me shake my head in disbelief, they really are crazy good.
The Pass snaps things more into tight focus, it is astonishingly clear, very spacious, plus I think it is more dynamically capable so does tend to be more critical of recordings. It seems to want to play a bit louder to get the same ‘impact’. I guess the Leaks are bringing just a little valve compression, but in a way that sounds wonderful. There is also a difference in noise floor, the Pass is absolutely silent, I can’t hear anything with my ear to the speaker, the Leaks have just a very tiny bit of hiss/hum as you’d expect, but nothing intrusive even in the extreme nearfield.
Bottom line: they are both seriously good. They just present a surprisingly different window onto what is happening.
PS The Leaks certainly demonstrate how mature power amp technology actually is. They date from the mid to late-50s and are not embarrassed at all in this company. I honestly couldn’t tell you which was the “better” amp, they both do things I really like and value.