I really think some of you guys are reading too many graphs. There's just no way to nail this issue down without taking into account the very critical variable of room interaction.
David, how much of a risk could it be, really? I mean, don't you think adding a second "quality" sub is going to "help" after you've had a chance to experiment with placement and EQ?
Just do it and get on with having fun instead of torturing yourself.
btw, the Behringer Feedback Destroyers - when used as a sub EQ - offer 2 channels of independent parametric EQ adjustment for a pretty reasonable price. If you want to measure and tweak.
