I just had a similar install where the subs were located under a small stage and were placed a few feet back from grill where the sound could exit. I was told that after the club got new carpet, the sub level was low.
In between the subs and the grills, the carpet installer had stored tons of scrap carpet and padding in case they had to repair the carpet in the future. We had the maintenance guys remove all the carpet and padding from in front of the subs. Problem solved.
I had almost the same thing a couple years ago at a church service call.
I could hear the rumbling of a subwoofer but could not see or locate where it was coming from, guy at the church told me it was under the stage, the stage had doors all along the front that opened into the dead space under the stage that was used for storage. They put the subs under the stage each by one of the doors and then closed the doors and the stage front along with the doors were also covered in thick carpet!
I mentioned removing those doors and replacing them with grating, framed perf metal or grill cloth. They were afraid it would be too much subwoofer then! By the way their system was setup with aux fed subs, I did try to explain it.