well, not an expert with JBL subs...
but a quick net search reveals a 250 watt powered integrated amp in a front firing cabinet.
so you might have several issues.
the room you showed is pretty 'squarish' at least in the pic you showed it seemed that way
imaging can definitely be an issue if your speakers are out of phase/sync with the sub, and if you don't tune them correctly to whatever crossover you set. with a denon receiver, you should be able to do this certainly. i imagine this is your biggest problem, but you might also try putting it in front center, too.
Thx for taking the time to do a little research.
I can't do much about the room unless I add bass traps in the corners. I also cant move the sub b/c idiot contractors put the coaxial wire in back instead of front as instructed. I'll run through the Denon AVR setup again and see if I can get a better sound.
if its a modern (and not the absoulte cheapest model offered) denon you can set inside of it the frequency range it sends to the LFE. set your sub crossover to the same value. adjust the sub volume incrementally till it blends.
your sub (couldnt find this online) may have a built in phase shift. if it does, you wont be able to correct that.
and finally, again, i think your sub as a 'front/side fire' will also sound much much better in front of you than behind you. downfires go behind you
