I’d have to agree with @millercarbon that, if you’re not going to get into the really big bucks, Nobsound springs (found on Amazon) are the way to go. I’ve got them under my 90lb speakers and under my subs. They definitely improve speaker imaging and soundstage and, I no longer notice the subs vibrating the soles of my feet.
if your system is good enough to reveal a lot of detail in the imaging, go for them. Otherwise, you might need to work on the weakest link in the chain, which for many people is the speakers.