I see the problem...the foam fill madisound includes is definitely not what should be used. You are not just dampening the cabinet, you could also be effectively increasing it's apparent volume (do a google search to find out why, I'm not the best to explain this to you).
The design calls for polyfill...polyester stuffing, like you'd use in pillows...3 inches deep from the back wall...good news is a $5 bag will do about 3 pairs of these speakers...go to Walmart and buy some, try it out....then, if you still aren't happy, we can try a few other things. I'm fairly confident the polyfill will help
Good luck.