I've ridden the B-17 trail not long after it was built in Marin (at Tamarancho) but don't know much about Marin bikes altho a friend of mine did pr business with the founder but never got to meet him. A buddy of mine from Marin and his wife and some friends of theirs rode the Mt Vision models for years (they're now on Ibis HD4s). I was working the TransCascadia race (shuttle driver) this last week and saw a new Mt Vision one of the racers had, nice looking rig. If the B-17 ticks off your requirements I'm sure it would be a good ride.
I'm still liking the 27.5+ too, haven't had unusual tire wear, altho I do tend to scrape logos on the sidewalls squeezing thru rocks that previous wheels cleared better
Check out some of the pics and videos of the
TransCascadia, the coolest mountain bike race/event there is.