I haven't seen this posted anywhere (except for the built-in speaker and the frequency response of some headphones)...and it is not mentioned in the specs in the manual.

What is the frequency response capability of the Treo 650's stereo output?

The Tungsten T2 is 160Hz - 19kHz, for example (a search for 'frequency response' revealed this Q&A tidbit on Palm.com).

The reason I ask is that using the T650's line output (via a 2.5mm-3.5mm conversion jack and a 3.5mm to stereo RCA connector, it seems that the high frequencies suffer a bit even when using good headphones. My guess is that there is either a cutoff or rolloff around 12-14kHz.

I am using Pocket Tunes as the player and a full-quality WAV file, ripped from a CD, and the highs seem to disappear a bit. If you try to bring them back using the EQ within PTunes, the highs come back a bit, but sound distorted. Perhaps this is a fault of PTunes. Or is it just a fault of the T650 hardware limitations?