I took the internal dimensions of the HTM-12 and plugging them into a room mode calculator: amroc - THE Room Mode Calculator (amcoustics.com)
It's obviously not the way the calculator was intended to be used, but you can click "do it anyway" and see low order modes from 285hz up past 800hz. Some of the modes stack up since the internal dimensions are pretty close to a 1:2:3 ratio. It might lead to extra scrap MDF from a sheet, but if the dimensions were altered just a bit then perhaps the resonances would have less impact without us having to heavily stuff the box.
If there are any resonances much below 285hz then it seems like they would be wall flex related and would have to be addressed with bracing.
It's obviously not the way the calculator was intended to be used, but you can click "do it anyway" and see low order modes from 285hz up past 800hz. Some of the modes stack up since the internal dimensions are pretty close to a 1:2:3 ratio. It might lead to extra scrap MDF from a sheet, but if the dimensions were altered just a bit then perhaps the resonances would have less impact without us having to heavily stuff the box.
If there are any resonances much below 285hz then it seems like they would be wall flex related and would have to be addressed with bracing.