Seems like an important tip to put into the calibration instructions, so you don't assault your ears and studio monitors with 78 dB of sawtooth wave at 40 hz. Imagine if you had the control room volume really high when you engaged the filter.
Anyway, seems like adding this sentence to the instructions would be useful:
"BEWARE! When you engage the HPF for calibration the unit will be putting out 76 dB of sawtooth wave at about 40 Hz."
2/2