Yes, the Polyflex diffusers are behind the amps. I also use two at the first reflection point on each side. I find them to be very effective. I use to have GIK diffuser/bass traps behind the speakers but the Polyflex are way more effective. Better tone, timber and soundstage. However, I still have a series of GIK bass traps behind the listening position.
The EM/IA is just one of those things you have to hear. I swapped it in, after a demo with it, for a Pass pre. Immediately, it just sounded more alive. Weird, really because it doesn't look like anything. I upgraded the ICs (Fideliums) and that was a big bang. Still, I'm curious what a really good tube pre might sound like. But, we can't have everything.
Thanks for the comment. Try those Polyflex diffusers!