This guitar piezo opamp buffer lacks any sort of bass, why?
The 1 Mohm bias resistor is far too low - it will load the bass signals and attenuate them much more than mid range or the treble. This happens because the piezo has capacitance and this capacitance is in series with the signal being produced. It might only be a few hundred pF and, for instance, 1nF and an external 1 Mohm bias resistor will form a high pass filter with 3dB cut-off at 159 Hz. Too high for a guitar by about an octave (low E is 82.4 Hz for example).

Try replacing the 1 Mohm with 10 Mohm and see if you get an improvement. Here's a circuit I found that uses a 10 Mohm bias resistor and so it's clear to me that that is your problem: -



You can carry on using the op-amp but note the use of the 10 Mohm resistor for biasing. Also note that C1 is not required. Here's another guitar piezo JFET amp that uses a 10 Mohm (R2) for biasing: -