I ended up using one of my Tektronix DM502s for the port tuning. I initially discounted this multimeter because it is "only" 3.5 digits but as it ends up, its low frequency response is flat down to to 10 Hz and its dB functionality is an asset in this application.
Below is a photograph from my second DM502 after I repaired it this past week; somehow the bridge rectifier for the +/-12 volt supply failed open, so I replaced both bridge rectifiers in the power supply as well as the three electrolytic capacitors. Since I pulled the difficult to replace RC4194 dual tracking voltage regulator to prevent damage, I replaced the crummy Texas Instruments edge wipe socket with collet pins. The two solid tantalum output capacitors are now in collet pins as well.
The markings wiped off of the first Rectron bridge rectifier which should not have happened. I was more careful with the second one. I guess I should have gone with Vishay.
In the upper right corner, you can see two thin wires going under a socketted IC. That is a thermistor under the transistor array used for logarithm conversion.
Incidentally, the subwoofer works fantastically. Last week I mistook wind noise in a vblog as distant thunder.