Why would you run them this low? One of the advantages of subwoofers is that you can relieve your loudspeaker's woofer from low frequency duty and possibly avoid using their port.
The mains are active, sealed, speakers and a combination of DSP, room-treatment and good-luck with positioning see the mains delivering smooth in-room performance down to 40Hz. I’m just using the sub(s), then, for the final octave where the mains don’t quite have the same impact.
It depends on your room's dimensions and construction materials. In many rooms, you're probably correct that the walls will be lossy enough / the room will be small enough that sub-40Hz frequencies will mostly escape the room and/or be pressurised. But it's not possible to generalise. My room is concrete construction with a minimum dimension of just under 5 metres (floor-ceiling) and there are modes present in the 30-40Hz region.
If your longest dimension is ~4m and/or/especially if it's a drywall construction, you may not have any problems there.
The best thing to do would be to take measurements if possible.
The room is large (7.5m x 6m) with a leaky construction (i.e., glass windows with drapes). I’ve done a bunch of measurements in the past and, looking back, can get flat performance from 20 to 50Hz with a single subwoofer plus a bit of DSP. Hence the thinking that the only benefit of adding another subwoofer is to get more output or, better put, to reduce the distortion for the same output via sharing the load across two subs. I’m thinking, though, that this isn’t worth the cost as the existing sub (an SVS SB-16 Ultra) does a good job, we don’t listen at very high volumes and I seem to recall that distortion at <40Hz isn’t particularly audible.