I've got a fairly standard cheap interior plasterboard/drywall wall that I'm going to install my W3s and W100 in. The wall is made from hardwood studs and 10mm plaster without any insulation.
My plan is to strengthen the wall without removing any plaster by-
1. Gluing (Liquid Nails) and screwing a strip of 16mm MDF either side of the opening on the inside of the plaster
2. Filling the cavity on all four sides of the the opening with expanding foam (Sika Boom AP)
3. Filling the screw holes and repainting the wall, which we want to do anyways.
This dodgy MS Paint diagram shows what I'm doing... The studs are red, the speaker mount black, the MDF yellow, the foam orange and the plaster not shown. There will also be foam behind the MDF.
Good idea, bad idea?
I think it will add a lot of strength and vibration suppression to the wall.