I'm considering getting a pair of Aura-Bass-shakers (they're only like $50 each) and attaching them to the under-side of my sofa....... the sofa has a metal frame that rests on the floor, so I'll have to attach them to one of the wood-cross-members, but nonetheless, it seems like a cheap upgrade..... I'm not sure how much I'll really get from the "affect" but friends have told me it would be "really cool to do" so for the sake of my guests' enjoyment, $100 isn't too much to at least give it a try.
Since I have a new sofa now that has reclining left and right seats...... I may start with one of these, and mount it under the center seat, and see how much of an affect that has...... I certainly don't want it to shake too much...... I want to enjoy films, not go on an amusement ride
This is certainly an upgrade/modification that doesn't really need to be planned on from "Ground-up" so adding it to an existing setup shouldn't be a problem for you.