With the full power output from our XM generator (A = 20 V), the output signal that has frequency below 10 kHz will probably be felt. Since that can be unpleasant and disturbing to a person, there exists an option in the "Program Options" panel to "[ ] Reduce Amplitude < (below) 10 kHz". The frequency here (number of kHz) is settable, so we can enter a lower number here, if we can tolerate a lower than that frequency signal (this way maintaining desired power level even at the low frequencies, if we can stand that). set / keep the voltage (amplitude) at such level which will be comfortable / acceptable for you.
To test the acceptable voltage level, I would start with first selecting a frequency set that contains low frequency(ies), for example, "Absentmindedness" (which has a single frequency of 5.8 Hz). Then start with (set) the Amplitude at 9 V, use ordinary square wave. Run/start that, hold the tubes, observe sensations. If all is fine (or nothing felt at that level), stop, close the channel [x], rise the amplitude by 2 V, test again. That should give you an indication about your sensitivity and what voltage level you can apply comfortably.