I have some vented speakers with an F3 of about 40 Hz in an EBS
alignment. I have severe room interaction problems, clearly audible
and confirmed by LSPCad: a VERY deep valley around 60 Hz. I can't
solve it without really inconvenient furniture rearrangements, so I'm
going to add a subwoofer (closed, Q=0.7, F3=37Hz) actively crossed
over to the main speakers with a 4th order LR filter. I will have to
use a xover frequency of about 100 Hz, above which the main speakers
behave reasonably well where they are placed. Luckily I have found a
very convenient spot for the sub where its response will be very flat
and it's at exactly the same distance from the listening position as
the main speakers.

Just for fun I checked what would happened if I closed the vents in
the main speakers and what I get is a Q of about 0.5, although the
rolloff starts slightly above 100 Hz so there would be a small dip
(less than 1 dB anyway) in the overall frequency response.

I understand the closed main speakers would have a much better
transient response than if I leave them vented, but I'm not sure about
their contribution to the overall system transient response. Since in
any case I'm filtering them electrically with a 4th order slope, does
it really matter how they are aligned below the xover frequency? In
other words, will the overall transient response depend on the box
alignment of the main speakers, or only on the overall system
response, crossover and sub included?

Many thanks in advance,

Carlos