Alternatively, I could try center 72 Hz, 1/7 octave each side (Q=5). That would cover 65 to 80 Hz. Then add my current center 64, Q=1.4 filter on top of it, +6 dB each, to flatten the entire range 45 to 90 Hz and bring it up to level. But that still would be +12 dB of boost, which demands 16x more power from the amp in this frequency range, and consequent phase distortion from a steep filter, and a -12 dB overall level reduction to avoid digital clipping.
Filters with Q up to 5 are certainly not "steep" filters. From what I know human ear is fogriving to phase variations with LF, less with MF and much less with HF.
If your filters have max gain of 12dB and you reduce overall gain in convolution engine for the same ammount (and you shuold, to avoid clipping) your amp and your speakers won't have any additional stress.
P.S. I still suggest you try it with 70Hz and Q=3 and see what will happen. Nothing to loose..