I haven't noticed any big change with burn-in. The bass did seem punchier to me. But I believe this was more of a brain-burn-in effect.
I'm glad you're enjoying them. If you find the sub-bass lacking(like I do), you could try a narrow lift of a few dBs at ~35Hz. They do respond well to EQ.
You are right the sub bass could do with a slight lift, so I done a quick adjustment and applied 2.5dB to 32Hz and 1.0 dB to 64Hz in Rockbox. It sounds better, more rounded.
Will try other settings to find optimal balance.