Wings Aloft on the southeast side of Boeing Field has had a 340 for charter for awhile so I assume they may have some knowledge. I see they closed their front office two weeks ago and moved one block further south near AeroFlight. Aeroflight has 8 twin Cessnas, but they will not do outside maintenance. I tiedown my 340 at Aeroflight in the summer, but have to find maintenance elsewhere. The mechanics that used to work at Galvins have moved to an old hangar about midfield on the east side after Galvins got out of the maintenance end of the game.
American Avionics is about the only avionics company left in Seattle and tehy are on the east side of Boeing Field north of Galvins. National Aviation is a good place for parts and supplies; they are across the road from Galvins.
If you like to change your own oil, Boeing Field has an oil dump and a few parking spaces on the southwest side just south of the Museum of Flight. There are buckets and receptacles in the shack next to a hangar.
Corporate Air Center out of Burlington http://www.corporateaircenter.com/ is where I did my prebuy and export C of A on the 414 that I just purchased. I believe that they manage 3 or 4 400 series Cessna’s. Nice place and Tim the owner is a good guy. They are also a Cirrus service center and have good comments on the Cirrus forum. I pick up the plane tomorrow so I’ll have a better opinion then.