I gotta agree with Pogre. Your Sub is not properly set up.
Yes, corners will amplify the sound, but the acoustics of the room will dictate how boomy and muddy that sound becomes. I don't put my subs in the corners for just that reason: otherwise very high quality subwoofage sounds like sh!t.
The cats who corner load successfully are using REW and likely a miniDSP module (like the 2x4) to eq the performance back to flat.
Regardless, this also goes with the idea that you are perhaps overdriving the Sub on top of it all. If you are physically getting your Sub to distort, you will get all kinds of room rattles and vibrations.
I'm a musician and LOVE Rock'n'Roll. I was the Barista responsible for playing MC5 and Iggy Pop at 6AM because you shouldn't start your morning out with somnambulant BS Folk cr@p! That said, I know how instruments are supposed to sound and I value accurate reproduction of the recorded event rather than trying to add my own distortion on top of what is in the production.
Anyway, I can play my rig loud enough to make my ears bleed and force me from the room if I choose, and my bass is still tight and accurate... with ported Subs.
My recommendation is to step back a little, look at your setup, and experiment some. It should sound just as good at -25 as -15, and if your gear can take it, at 0 and louder.
Finding the right spot for one sub is easy, two gets trickier, but if you value clean output, taking the time to look for those spots will pay serious dividends in SQ. Same will go for your Speakers.