I don't know what's caused your high frequencies to go bad in the player, other than a possible corruption of the flash RAM and/or hard disk which would cause the equalizer presets to be all screwed up. Check all the EQ presets, as well as the Hijack bass and treble adjustment settings.

Regarding the playing-by-itself problem, the current (not fully understood) wisdom on the topic can be found here.
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Tony Fabris