However, if your headphones have a high impedance, then things balance out, the bass won't roll off as much and the rather loud headphone out of the Cowon S9 will be of very good use compared to quieter players.
Using an amp will solve the bass roll off issue in most cases.
Also, the Mach3Bass setting, although not a complete solution to that problem, is still very useful since it raises the frequencies around 60 - 80 hz - precisely where the roll off usually start with lower impedance headphones. But it won't do justice to even lower frequencies. Still, you'll get that rythmic bass section that's usually lacking because of the roll off issue.
But in the end there are other things under the label "sound quality". For instance, the Cowon S9 seems to handle dynamics very well. It's also got a rather liquid presentation - something I personally value, and of course those BBE enhancements, which are not that well considered by purists but which can, if used very moderately, give you the little difference here and there you missed on other sources. It sill is a very good player, and I feel it's actually a great one with higher impedance portable phones (Shure SRH840 for instance, even though I feel they still benefit from amping).
