It is effective to have the amplitude set at max, even with Spooky boost v2.0. The point is, it is not the voltage that heals, it is the current. Because some body resistance is high, with higher voltage(amplitude), it will ensure adequate current passes through your body.
Again, these voltage levels are not dangerous, but there's no need to make yourself feel uncomfortable when lower frequencies are being run (therefore a testing from lower to higher amplitudes first). The point is that you will most likely not feel anything when the signal has the frequency above several kHz, even with the signal's amplitude/voltage of 10 V (20 V set in Spooky).