Here's the place where I take my 850. I've been going there for years:
A & M Foreign Motors
181 Mcgrath Hwy
Somerville, MA 02143-3417
Phone:(617)776-1760

It is a stone's throw from Lechmere/Kendall Sq. area and they'll give you a ride to the redline or greenline T if you drop it off in the morning. It's a relatively small shop, staffed with pleasant, helpful people.

Between me and the rest of my family we've taken 5 or 6 volvos (turbo and non) there over the years. They are very familiar with volvos, good at what they do and pragmatic about repairs -- for example they recommended converting my Dad's 760 from nivomats to regular springs and shocks when they needed to be replaced. They have often told me NOT to fix things, or at least to wait until it really needed it. Once a part failed 10 months after they installed it and they replaced it for free. Perhaps most importantly the repairs they have made for me always fix the problem, unlike some of the horror stories I hear from friends who go go elsewhere. The place I went to before A&M always held everything to the standards of a new car and just replaced parts rather than diagnosing the actual problems.

They've also done pre-purchase inspections for me with very consistent results -- the only time I've regretted a car purchase was when I didn't get it checked there first -- and performed the last rites for a few of my cars when they could have made money by telling me to keep fixing them. I've taken a VW, a Honda, a Saab and a Chevy there for repairs, and my father gets his Porsche fixed there as well.