I had dual 12" subs that took 5 or 6 different placement configurations before finding a good one. For a while i thought i had waisted my $. Then i found that "sweet spot" & it was a complete night & day difference. So yes the OP absolutely needs to find the right location/s. My room has 1 REALLY good spot & a 2nd decent spot. So while he is undecided on which sub/s to buy he should start off with a bass crawl of course. This should get him headed in the right direction. Like mentioned this is 100% FREE!!!!
Once the spot/s are found he may fall in love with what he has or find out yes a new sub/s are in order. Im thinking one sub in front right & the other in the rear left (or vise versa). That is a guess of course. Or side midwall placement. Or stacked in a front corner. Thatd give the most output!!
What would your budget be?
SVS PB12 NSD $749
HSU VTF-15h $879
HSU VTF3 MK4 $699
Outlaw LFM-1 EX $649ish
To minimize square room effects id highly recommend the route i took. Get a good sub to place in a good location across the room to handle 50hz & below. Any of the obove subs would be great for that. Then add an HSU MBM-12 (mid-bass module) & place that very close to your seats. This will handle your speakers crossover point, ie 80hz & it naturally rolls off at 50hz. That is where the other sub would pick-up at. Being close to the mid-bass gives you incredible slam & the low bass waves have distance to breath & sound extremely smooth & powerful.
There is some debate over this way of splitting the bass but im a believer & it would be my personalmrecommendation to you.
If you are uneasy about buying an ID brand, remember, you have 30 days to return it if not happy. But trust us SVS, HSU & others are VERY good subs. Tried, tested & with experienced refferences. Besides DIY its one of your best bets!!!