How thin and rigid is the thin wood? If not rigid/solid enough, the vibes won't be coupled effectively.
I had similar problems because like you, I have a couch with recliners. I was driving 4 pro's with an Adcom 555II ~160watts per channel(2 shakers/channel). I was cranking it way up to the point of seeing overdrive warnings on the amp! Still, I could barely feel the shakers.
My shakers were screwed into the wooden part of the frame, but my guess is that the vibrations weren't coupled well enough through the rest of the chair to the seat. On my couch/recliners there doesn't seem to be much in the way of direct contact between the wooden framing and the metal reclining mech of the seat.
The challenge was to find a decent mounting place close as possible to the bottom of the seat, and still have clearance for the reclining mechanism.
I found that underneath were 2 metal railings running directly beneath the seat from the front of the sofa to the back. They were even nice enough to have small holes already so I didn' t have to drill. A 3/4" of plywood cut to measure, secured with drywall screws into these rails and I'm all set. No problem getting the shakes now.
My point is that for my situation, attaching to the wooden frame didn't work well. I found a way to attach a piece of plywood to the metal part of the seat frame. Thats my best result.
However, the seat is a bit less confy because when you sit down, you make contact with the plywood that is under the cusion. Not much you can do about it in my case, but I wouldn't have it any other way. Good Luck!