Thanks Al & Shadorne. The point I was sort of alluding to is that it seems to me that attempting to do level matching by ear, we would end up with the sub's level set too high. Thus, the sub would call attention to itself and/or sound boomy.
Yes, now I see what you were saying in your second question, Bob. But I think that my previous answer still stands. Test tones would give that result, to the extent that perceived equal loudness can be judged accurately between a 1kHz tone and a 20 or 30Hz tone. Unless, that is, the tones are played so loud that the "perceived equal loudness curves" become flat, within the bandwidth of the sub, but I doubt that anyone would want to listen to a pure 1kHz sine wave played that loud.
But music would not give that result, because what would be judged would be the timbral accuracy of the instruments, and the settings that result in best timbral accuracy, definition, etc., presumably are those that place fundamentals and harmonics in proper balance, which presumably is what would correspond to measured flat frequency response.
Regards,
-- Al