@ghdprentice
MacIntosh is probably stating the specs within the audible audio spectrum, which is 20hz to 20khz. It undoubtedly produces output well below this when fed appropriate material.
All mainstream manufacturers state their specs well below the audio spectrum 20Hz. It's subpar that McIntosh can only state 20Hz-20kHz. It's a premium $10K for subpar performance.
I asked McIntosh and they stated it continues to produce audio below 20Hz. But they cannot make any specific claims, such as -3dB at 15Hz. Therefore, you have to conclude that it cannot even do -3dB at 15Hz. If it's -6dB, it's bad. For reference, the C55 preamp claims -3dB at 15Hz. Even this isn't stellar but if the preamp is used just for music, it's not a major issue.
Regarding compensating the subwoofer, you can't just increase the volume knob on a sub because it increase the volume for the entire utilized bandwidth. If you have a stellar room control software, it can be more precise but will likely not be enough to compensate for the shortfall. For example, many amps state up to 20kHz but it falls off hard after 20kHz. I can EQ the bands above 16kHz +12dB but it won't do anything since the amp is almost starting off at zero dB, 12x0=0. Therefore, I don't think you can make up for the shortfall by boosting the EQ on subwoofers.