I received the Ref 150SE, dropped it in, and was not initially blown away.
It has ~600 original hours. It has always been an SE.
I let it play for about 10 hours, still wasn't taken by it.
I put the Ref 75 (non SE) back in. It sounded sweeter, but thin and tinny/splashy by comparison. Put the KT150s in the 75, it sounded less sweet, less splashy, but firmer bass (which can sound like less bass), also less billowing (bellows) sound. It gave a bit more body to the Ref 75.
I put the Ref 150 SE back in, and realized how good it actually sounded. Plus I think it hadn't been played in awhile, so I think it's still having a bit of a re-breaking/settling in.
Short story: Ref 150SE is now beating the Ref 75 nearly across the board.
NOTE: It was critical to have isolation under the digital source to get sweetness back from the system w/ the 150SE in.
If anyone is interested in deeper, specific, more critical analysis as I go along, I'm happy to check back in here.
I'll likely be playing and swapping through the weekend.
Thanks!