Right a few more songs on, and I feel the bass is pretty decent after all. I think its just because I've actually now played songs with some decent bass in them. They do seem to have slightly more impact and volume at the bottom end than the DT531, although that headphone is not lacking either. These however, do have good PRaT, possibly on par, or better than anything i've yet heard, at least, anything with enough bass to be easily heard without cranking the volume.
I've not had any issues with sibilance - they are a touch bright perhaps, but certainly nothing too bad. I guess its system dependant, but overall they are seeming fairly well balanced, and great fun to listen to so far. I can already say for sure that these are the modern day groovalizer, the one with the closest ethos i've yet heard to a well driven DT531.
I cant comment on driving it with ipods and the like as i've not yet tried it, but i will say on the below rig (one which i regard as none too shabby in any respect to be fair) its performing beyond what one should probably expect for its pricetag.