I was using my Fryette PS-1, or at least trying to. Admittedly, I didn't really care for it through the Fryette and had better results through the power section of my amps, which ultimately defeated the purpose for me. I thought about picking up one of their heads, because everything I had just sounded better through my buddy's SYN50, but ultimately just ended up selling the system as my main goal was to simplify my rig and adding another power amp or head was the opposite of what I wanted to do. That said, I did try it through the power section of the Mark IV and I still had similar feelings with it.
I've heard time and time again that the power amp makes a huge difference on these, which I definitely tend to believe... But that's why I got rid of mine. The whole purpose of the module system is to have a smaller package that can get most of the way there with the promise that the power amp is the least coloring part of the sound, so almost anything will work well. Since that definitely doesn't seem to be the case, it's novelty was lost on me.