optimal carrier tone frequency

[rowatc]

Currently, the software performs AM modulation with a 1kHz carrier tone to allow the electrodes' signals through the high-pass filter on the device's audio jack.

Would adding less than a 1 kHz carrier tone require less power, and distort the signal less?

We don't think so: we think anything less than 650 Hz would not be physiologically meaningful; a higher frequency carrier signal samples the neural signal more finely. (The Nyquist rate puts a lower bound on the sample rate.)

You can experiment by changing the carrier signal in software.