You load a profile from AutoEQ, it tells you “Preamp: −7.4 dB”, everything goes quiet, so you turn the amplifier up and now something distorts. This works out the correct preamp for your own filters and, more usefully, what that costs you in amplifier power.
The number most people guess is wrong in one of two directions. It is not the sum of your boosts, and it is not the largest single boost. It is the highest point of all your filters added together — which is a different number whenever two filters overlap.
Filters in a chain multiply, which means their decibel curves add. Two boosts that sit far apart never meet, so the highest point of the pair is simply the larger of the two. Two boosts that sit close together pile up, and the peak lands somewhere between them, higher than either but lower than their sum.
Take three filters: +6 dB at 100 Hz, +4 dB at 120 Hz and +5 dB at 3 kHz, all at moderate Q. Adding the gains gives 15 dB, which throws away nine decibels of headroom you did not need to lose. Taking the largest gives 6 dB, which clips. The true peak of the combined curve is +9.68 dB at about 107 Hz, and that is the number to use. Move the second filter to 1 kHz and the same three gains produce a peak of just 6.0 dB.
AutoEQ does exactly this, then writes Preamp: −9.7 dB into the config file. The extra tenth in its README is a safety margin, not a different calculation.
Cutting everything by the preamp and then restoring the volume means the amplifier now works that much harder. A −9.7 dB preamp asks for 9.3 times the power and just over three times the voltage. On easy headphones this is invisible. On a demanding pair driven by a dongle that was already near its limit, this is the whole reason bass EQ makes things sound worse instead of better — you did not overload the driver, you ran the amplifier out of volts.
One honest note: this is the worst case, calculated for a full-scale tone sitting exactly on your highest boost. Real music almost never does that, so treat it as a ceiling rather than a prediction. Filter phase can also push the true peak about a decibel past the magnitude peak, which is why leaving a little margin is sensible.