Wow, that's a quite noticeable difference, thanks. I hadn't really tried putting the cups that far back on my head (and ears scraping the front of the pads) but it definitely makes the bass less "boomy" on songs that don't need it. I'm not sure if it helped the imaging at all (maybe a tiny bit?) but now I know when to move my ears to the front. Any other secrets?
Have you done the HD650 comfort mod to the headband? You buy a Sennheiser HD650 headband and replace the weak stock one.
Also I swapped pads to Mr Speakers Alpha pads. Better isolation and overall better sound and tighter bass.