You could try damping the low end but you'll need some pretty thick material to really affect bass frequencies. If you want to go that route, you may want to read up on building bass traps. Here's one
link to get you started. There should be lots of good information in this sub-forum as well.
Another way to go would be to either EQ the system or change the speaker crossovers. EQing would probably be easier. Changing the crossovers could be as simple as using smaller resistors in the tweeter L-Pad circuit but could involve re-doing the BSC as well. It would take some careful measurements to know for sure.
If equalizing is practical for you, that would probably be easiest. Otherwise I guess try absorption. Have you tried something simple like sticking pillows or cushions on the shelves?
Jim