Hi,
6th order vented involves building a 2 order high pass
into the feedback loop of the subwoofer amplifier.
Its not complicated if you know what you are doing.
(The inverting input can be treated as a unity
gain point for standard unity gain filter design).
Typically 6th order is a vented box tuned much lower
than normal, with a peaking high pass coincident with
the vent tuning frequency to bring up that region.
Many alignments are possible, hard to say which is best,
but IMO maximally flat is not a good idea at all, IMO its
best to aim at a slow roll-off of say 3dB/octave* and let
the relatively predictable room gain do its work.
rgds, sreten.
* Down to the vent tuning frequency. IMO a high pass
with Q of about 2 (+6dB peak gain) is about right.