This is certainly a room mode cancellation (and is extremely common for smaller rooms). It’s a function of the speaker positions in the room as well as the listening position.

If you’re unwilling to move them out into the room, you can try moving them left/right as well as move the listening position left/right/forward/back to see what it does.

Bass traps will certainly help, but they’re not going to magically fix a 30dB null.