I run my pair of SVS Ultra subwoofers flat. I like deep, natural sounding bass, not excessive bass that should not be there.
Of course, if you listen at very low levels, you might want an increase, but to do loudness compensation right, you would need to be boosting frequencies above just the subwoofer. For that function, using a device (like a preamp) with a loudness compensation circuit will give the best results.
Also, if one's subwoofers are not going low enough, one might miss that deep bass and want to boost it to try to make up for the bass that is absent. That, of course, never works out quite right, as one still is missing the deepest bass and one is getting too much upper bass that way.