I have another theory. Perhaps your HSU sub was not inert and as a result may have been vibrating. Now if that HSU sub was directly coupled to the floor (assuming not a concrete floor), could cause the floor to resonate. The table at 15' distance, which is still coupled to the floor could vibrate.
Yes, the table could vibrate sympathetically with the floor. But floors don't vibrate from coupling, they vibrate in response to the sub's audio output.
Is it normal where you turn the volume up high you start to lose the SPL of the bass? Like when I turn it up the ULF gets less and less.
If you're saying what I think you're saying, yes. The driver will run out of excursion at a lower level in the lower end of the passband than in the upper end. That means there comes a point where turning the sub up louder won't get any more volume in the ultra lows, but it will get more volume in the upper end, changing the system response.
Well after reading this thread. I decided to mess around with my positioning of my xv-15 again. It's crazy how big a difference a few inches can make.
Subs are like a business: The three most important factors are location, location, location. Where yours is concerned if it's not aimed into the corner (I can't tell from the picture) it would probably work even better if it was.