Just bought a pack of these - http://www.wickes.co.uk/Grey-Slate-Tiles/invt/235443 - and I can definitely recommend them. I have one beneath each speaker, and the difference has to be heard to be believed.
My speakers are either side of the gas fire, but being an old house, there is a stone hearth beneath the fire, but it doesn't extend to the speaker locations. The floorboards around the hearth are pretty ropey and uneven (despite numerous attempts to plane off the high spots), and the whole floor has been sheeted over with hardboard, which I think is what was killing the bass before (the speaker spikes connecting with the hardboard, but not necessarily the floorboards - knowing how hardboard likes to take its own shape regardless of any fixings used *-) ). Anyway, with one under each speaker all is good. I'll try doubling them up shortly, and see if things improve any more.
I'll try refilling the stands again. When I filled them before it killed the soundstage, so I emptied them out sharpish. Maybe now they've got something more stable to rest on it might improve things.