If you can get your hands on some leather pads (and make sure the earpad cavity is well sealed), you might gain in bass region (unless the rolloff is due to your microphone and not an actual illustration of back wave concellation due to transmission through the porous pad). However, whatever you may gain in the LF, you'll get even less sound absorption so the MF/HF response will get even more tilted up.
The most effective method would probably be the foam layer agaisnt the driver on the ear side. It might quickly get dark sounding (and the soundstage width suffer), but surely it will tame the highs. With the very large diameter perforations, I suspect you get little damping from the thin air layer, which may seem like a good thing at first but end up hearing the diaphragm's resonances.
Awesome DIY stuff though, really enjoying reading you guys updates!