Concrete blocks and 2x12's. (In 40+ years I've never had any success with any of the tweaks I've tried.)
Seriously for my simple system I use a leftover 12 inch by 43 inch piece of shelving that I varnished and added four spikes to sit on the floor between the speakers (pulled roughly 5 feet from the front wall), similar to an extended amp stand. This replaced a small rack (Ikea night stand) and very nicely opened up the soundstage.