I'd say I've looked at a few graphs as well, but I can't remember one where 30Hz was affected by the mains crossed over at 80Hz. Not saying there aren't any... just saying that it's most likely be a very small percentage.
Either way, we've been making suggestions in this way since 2001, measure the sub only... equalize it and then measure with the mains to see the interaction, since most of us only have the option to eq the sub. Maybe we've been doing it backwards for 6 years
... but it's been effective.
