Alright car people, I'm lost on this one...
For the last 45,000 miles or so I had a set of Michelin Symmetry tires that were fantastic. They wore out, so I purchased some new ones - Mastercrafts this time. Immediately noticed a vibration, so I went back to the shop and had them re-do the balance. Still had the vibration. They look at the tires while spinning them, and two are visibly out-of-round. Two new tires in - both also visibly flawed. At this point I pay the difference and have the shop get me a set of Michelin Primacy MXV4's. They install them and while it's less noticeable, there's still a vibration at about the same speeds (~30, ~60, and anywhere above about 75). Took the tires to a different shop to have them try to balance them. They do, tell me that the tires look fine but they were out of balance. Pay them, go on my way. Still has the same vibrations. The vibrations aren't felt through the steering wheel, but rather through the seat, if it's of any help.
I never had any vibration problems with my old Michelins, but ever since I tried to replace them, I can't seem to get rid of it. Now I'm becoming concerned that the tire shop damaged something during the installation of the new tires.
So - does this still sound like a tire balance issue, or is there something else I need to check? Any advice is appreciated!