B&W 800D or ATC 150 Passive

I'm sorry to tell you that with your lovely Mc combo,you will hear 10 or maybe 15% of the real potentials of both the speakers you mentioned.
I would personally go for a pair of actives 150....you can always drive them with an high quality valve preamp ;)

thanks for the comments. I suspect you are correct that the active ATC's are better than the passives.

I like the sonic flavor of "tubes" and the vintage MC275's/C22 with the right tubes hit the mark.

I am running a pair of ATC passive 150's now; deal demo units. The mid and high's are right in there with the B&W 300D but the full low range is much better with the ATC's.

this will be a hard choice. thanks again.

I'd recommend you consider the active ATC's and the all-
sonic(speaker + poweramp) solution this represents, compared
to the McIntosh MC275's + B&W's or passive ATC's.

Active ATC's to my ears make for a very worthwhile sonic
advantage compared to their passive siblings, even where
extremely capable amp constellations are in use. Whatever
sonic "flavor" you're after I'd seek outside the
influence of the (separate) poweramp, especially in light of
the inherent advantages that springs from the active ATC's.