I have 2 subs right now. The older SVS PB13-ULTRA and an SVS-PCI 20-35 or something like that. Currently the PB13 is up front in between my center and right channel. Ideally with a room this size where should the second sub go? I have a Pre/Pro with MultiXT32 and dual sub outputs for EQ.
Read this article http://mehlau.net/audio/multisub_geddes/, if you don't have measurement gear it will be hard to implement multiple subs effectively, you can try and follow best examples, but every room is unique and multiple sub placement is a science not an art.
You want the 2nd sub to fill in The lows/nulls of the first.
And the 3rd to fill in the lows/nulls of the combined 1st and 2nd response, etc