Room treatments behind or between speakers?

Nothing should go between your speakers except for the amp on the floor.

GIK told me put mine either side of the flat screen (they were working from a schematic I sent). The panels ended up directly behind (and mostly above) the speakers. These were the first panels I installed and they definitely made a difference. BTW - a while ago, came across a thread here on A'gon where a poster was citing an audio expert who claimed benefit to having a somewhat reflective/not fully damped surface behind/between the speakers. Don't recall the details well enough to explain the rationale. It's a school of thought that didn't seem particularly "intuitive" to me.

It's room dependent, and Milpai's treatments are a good example of absorbing bass then getting better imaging by using a middle panel.

Most acoustic products are made with fiberglass. If you're on a budget, you can buy fiberglass panels (not the big rolls), and use them for room treatments. If they work out, you can cover them with fabric so they look nice

A lot of people put diffusers on the wall in between speakers.