Room treatments behind or between speakers?

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.

Use mirror method along the wall to locate position of the treatments.