In the past I've experimented with ordinary paving slabs available from B&Q and the like, resting on three screws (so self levelling) that go through the carpet into the floor. You can tart the looks up to a certain extent, but the main benefit is that it's low cost, so you can see if the 'mass' approach works for you in your circumstances. If yes then either spend more time getting them to look better, or more likely go out and spend money on a nice granite (or slate) solution that looks and sounds the part. Oh and the screws don't damage the carpet, so S/H WMBO should be OK.
Basically you get to play for under £10. And it's amazing how useful the slabs can be in the future; I once used 2 under a Bose Soundock 10 to tame the bass. But boy what bass it then kicked out!