From the dimension you gave your fist room mode is 37 Hz, and the second is 75 Hz. That is close to what you observed.
The sub though is barely a sub, in fact I would not regard it as such, but a bass module. According to the specs the F6 is 27 Hz. It is an ABR design, and they roll off at 36 db per octave. So F3 is going to be north of 30 Hz. The speck sheet is not very informative except to give the 6db down point. So we don't know the design of the driver, but I'm suspicious from what you measured is has a high inductance, which would make the sub a one note special, like you see in that graph.
So, your best bet is to try apply a lot of Eq. Now there will be severe limits to improvement. The top of that peak coincides with the F3 of that sub design given -6db at 27 Hz. The sub is rolling off at 36 db per octave below that peak as predicted from its ABR design. Now since an ABR is a tuned design you can not apply Eq below F3, as you just get useless cone excursion and damage the driver. You can apply Eq judiciously above the 34 Hz peak.
I fear your options for improvement though are somewhat constrained.