To the OP- if you want to know if a wall, ceiling or other object will affect the bass frequencies, hit it with your fist. If you hear a low thump and especially if you hear it vibrate, insulate it. If you have access to the backside, you can add mass to it. If it vibrates, it acts like a drum head.
This works for removable ceiling tiles, too. When I worked for an audio store, they had a demo room with their 'high-end' brands that happened to be directly below the air handler which made a low, rumbling sound. Since it was always operating, it was always rumbling and while I was doing some other work, a couple of the sales people asked if something could be done to remove the sound because they were having a hard time selling equipment- the bass response was terrible because it was being masked and canceled by the air handler. I asked the mall's maintenance crew if the fan speed could be slowed and they did that- it helped, but didn't cure the problem because the room had 2' x 2' suspended ceiling tiles. Adding another layer of tiles and 6" fiberglass batts above made a huge difference.
If the top of that bar has any gaps that prevent the internal space being sealed, fill it. The air space inside can resonate, too- if you blow across the mouth of a bottle, it makes a sound and filling it changes the frequency. The space inside of the bar can work in the same way, but at a lower frequency if it's not insulated.