Do you not have access to a Radio Shack? Their 35 buck (US) SPL meter is a gem to use along with test tones. It's very difficult to be accurate by ear.
If I had to guess, your sub might be pumping into a null, though that seems odd in a corner, though I understand that square rooms are subject to all sorts of weirdness.
The Everest book should be your new best friend, before you spend any other money on this- though seeing if small adjustments in the subs position then trying your sweeps would be worth playing with- in a square room, I'd guess that you definately don't want it to be equidistant from both walls.
