I know you said Gaia and Townshend are out of budget, but if they ever come into budget please do trial them. I found both upgraded my speaker in the bass department to an unexpected and astounding level. In Munich Isoacoustics were demoing their Gaia in the noisy main hall on an open stand. Two sets of speakers were running next to each other, one on Gaia one not. Even in this noisy place the differences were so apparent, as they switched between them A/B fashion.
I have tried thick foam in the past, under each corner, folded over so that the speakers gently rock on the four wedges. That did something and is nearly cost-free. Worth a try.
In the meantime, definitely try the spikes if you have a set as they will probably help a little.