Did you do the math on the density or just wing it with the material? I'm asking because I stuffed my first panels way too tight and they ended up reflecting more midrange than I wanted. It took me like a week to figure out I had to pull some insulation out and leave a bit of air gap behind the fabric. Also, if you haven't already, definitely check the seal around the edges where the frame meets the wall, that's where I lost a bunch of low end absorption on my first batch. A little bit of weather stripping or even just a tighter fit makes a surprising difference for how much bass gets trapped.
My buddy Mike spent like two weekends building panels for his garage studio and stuffed them with that heavy rockwool until they were hard as a brick. He couldn't figure out why his vocals sounded so dead and boxy until I told him to pull half of it out and leave an inch gap behind the fabric. Now he swears by the trick and even put a couple thin layers of foam behind the frames for good measure.