the bass rolloff is more of an issue with the wolfson DAC then the actual tunning of the player, aswell as the filtering circuit on the HP out,
i would really like to see more DAP manufacturers drop wolfson and start using products from AKM and higher end cirrus stuff,
as for the bass issue between apple and cowon, i found the bass on the ipods very smudged and uncontrolled, its all over the place and lacks detail, basicly fart noise,
it might be a bit rolled off, but its more detailed and refined on cowon players,
i like to describe bass as a downwards facing cone, you have a impact point, where the pick touches the cord or the stick touches the drum, and for some, feeling that impact is more important that getting that shockwave that as a friend of mine put it "pushes down his food", bas should have impact, defenition and a dry start point where you feel the instrument react and the sound expand to a force, forming that cone,
if the cone is too short, the bass is too shallow, theres no impact, its just like cheap commercial hiphop music, on the other side a thin long cone will give you a weak impact point leading to a weak wave expansion which will leave you with no force and no enjoyment (animic bass)
alot of people make the mistake thinking that the bass is the expansion wave and not the impact, thats the higher mid bass hump alot of players and headphones have, i might be a purist or a fanatic, but thats the way i see its, impact, expansion and then disippation,