karma is a good output but not the best surely.
all outputs have problems: clip, sony (newer models only since 600 series), sansa clip and fuze, old iriver 140 and 300, ipod shuffle 1g have no roll off of bass with LOW OHM phones.
they however have varying degrees of success with producing stereo image and often, other than clip (from what i have heard), they all hiss. i have heard all of them except the clip and they are all hissy : some to the point of being almost impossible to listen to (iriver and sony, but iriver nearly the worst i have heard).
that is with low ohm sensitive phones.
with larger phones with lower driving requirements, the amp will not be overburdened and bass will not roll off. most phones above 100ohm should not pose a huge problem for portables to drive, but they will not be loud and perhaps lose a bit of an edge. they will however not hiss even with iriver models.
all my portables (meizu m6, cowon d2, sony 600, 800, ipod shuffle 1g, iriver 140, ipod touch and my kenwood, sony, panasonic and sharp md units) all sound good. for their tasks: equally as good.
meizu: hisses but has warm sound with great eq.
d2: good with large phones but poor with low ohm phones.
ipods: great stereo image with typical bass roll off, but gapless.
sony: hiss but good bass and decent stereo image.
iriver: no bass roll off (only older models) but hiss like mad.
md units: underpowered, overamped lows and highs but no hiss.
they all sound wonderful but the only ones with true native gapless are ipod and md.
this thread will only get votes for certain players, will not find truth about which is best as each player has a certain place where it will fall on its face for performance next to others and certain points where it will cause heads to turn.
