why do the access panels on a tuba24 need to be air tight?

[Colorado Hick]

I would think that having venting behind the speaker would make it more efficient and not impact the sound coming out the horn. In my (admittedly unscientific) tests I could not hear a difference with the covers on or off.
My apologies if this has been covered before...

[Radian]

Primarily to maintain the box tuning below the horn cutoff frequency, among a few other design considerations.