2 subs?

Google 'room mode calculator' and look for the Harman International White Paper on multiple subs.
'evens out the sound' is not quite exact. 2 subs can be located to excite room modes differently and in that way 'even out' the sound. The 'peak' nodes of one are where the 'low nodes' from the other are. The result is 'even'. People that tell you you can 'equalize' the problems away are non-starters. Just like a successful business Location, Location, Location. If the subs are located both the same, say mid wall on opposite sides of the room, you may not benefit from doubles. But, if you can manage some asymetry you can perhaps work it out.
I have the good fortune to have a listening room with 8 walls and an asymetric vaulted ceiling. I have a single, corner loaded sub which provides mucho even bass and is both musical and good for HT. AND, as a bonus, it is not a megabuck sub, but a modest HSU research VTF-2/mk3.

Work with what you got, first, before doing anything drastic which may PO the significant other.

I'll check out the Vandersteen site to see how well it agrees with what I read at Harman.......

Don't throw any money at the problem until diagnosed. Send everyone to the movies and start moving the sub and speakers until you get a result, even if you can't leave them there. Than you'll know.