I play pretty high up the neck at times. My high E string is tuned exactly to the pitch of the highest E string on a standard guitar. It is a .013 gauge.

When I play the 27th and 28th fret (G and Ab) the output of the POD decreases by half volume. Above and below this position, the string has full output. I tried these same pitches (very high up the neck) on another string. Same result.

This response is just like what a bandstop or notch filter would do. I suspected it was in the digital domain of the circuitry, but it sounds and behaves more like an analog section problem. I brought my guitar to a store and tried a few new ones. Same result.

These were designed for stamdard guitars which never have 27 frets. They do have overtones in this range, so there is something missing.

Good luck, and I have enjoyed my XT,

I would suspect, though, that the 32kHz sampling rate would just cut off frequency response above 16kHz and NOT allow signal above that. This response curve is a notch, in that I get a strong signal above AND below this pitch.

I could see this being a "digital" issue in that a typical passive bandstop/notch filter would not have this narrow of a bandwidth. This isn't telling me what kind of digital issue, though! As of yet, I don't suspect the sampling rate. Maybe some sort of 180 degree out-of-phase alias cancellation?

Many times you will find filtering in an op-amp circuit for filtering frequencies above intended use. There could be some unintended interplay, since the manufacturer did not expect any "regular" guitarist to be at the 28th fret. Usually they only go to the 24th.

Even so, it is in the audible range, so it seems there should be an effect to harmonics of the 14th fret (which I can't seem to detect).