HELP!!! I'm losing an odor battle in my new-old house .

I recently purchased a 1965 quad level house in Michigan and I love everything about it minus an icky smell that seems to come from the vents. I wouldn't describe the smell as musty but more a mix of stale/old wood with a funky twist. I've been hunting the source of this smell for about a month to no avail and was hoping some of the experts on here might be able to offer new leads!

The smell exists mainly in the first floor (which has absolutely nothing in it), but I can get a hint of it pretty much anywhere in the house. I can get a whiff of it if I stick my nose down a vent when the air is running (with either air conditioning, furnace or plain old fan use). It doesn't appear to fluctuate with weather although the house has bad and less bad smell days. There is no mold anywhere in the house and the basement and attic are dry. Here's what I've tried so far:

-Leaving windows open for days on end
-Changing out the furnace filter
-Vacuuming out all of the registers/vents (found lots of random stuff including a cache of socks, nothing smelly though...)
-Ripping out the carpet from the first floor (right now I've just got a subfloor for flooring!)
-Kilz'ing the first floor x 2
-Using Room Shocker in the entire house
-Putting out baking soda everywhere (yeah, pretty bizarre I know but I'm desperate)

I'm not sure what to try next (perhaps exorcism?) and would be forever in the debt of the individual who could grace me with a solution to this formidable problem.