I assume phase flattening below 100 Hz mentioned above does not eliminate the need to restore the "Missing Octave(s)"?
Was your re-EQ work that provided 6 dB of total end-to-end correction done to all your music files saved on disk? If so, is it possible to re-EQ the original music once to correct recording mistakes and save those changed files as the new "master" copies? Then if any further tuning needs to be done to the music due to a DSP change, room treatment, new driver, or a new amp, use DSP to make the adjustment once for all music being played?
Mark