One suggestion, read Jim Smith's articles about adding subs:
In fact, you dont need a pro RTA, I did it with an Android app called Sound Analyzer App wich is enough for the job.
You have to find the flatest bass response possible and doing it by ear is not an easy job.
The app helped me A LOT to find the best location for the subs (among the ones I've had due to decor), the best direction, the correct phase and a good crossover-volume level adjustment.
I've just bought a couple of medium quality subs and after following Jim Smith's guide for proper subs set up, with an RTA on some steps, the 3D sound and bass energy Im getting from my system is incredible!
And I still have to keep on tunning them for better sound and then run Dirac Room Correction to get full perfomance.
Its incredible how the mids and the highs have improved with the subs.