I recently finished building these 4th order bandpass subs, which I designed to complement my Geddes Abbeys.
The port opening is at the top end, covered by grill cloth. There is a ¼” layer of cork between the sonotube (14” diameter) and veneer to smooth out the surface and to add some extra dampening. The drivers are B&C 12TBX100s (same as the Abbey woofers) and are mounted vertically with a 4” port and a fairly small front chamber. Here is a picture of the baffle and port assembly:
I am using a DCX 2496 and external amps to drive them. I built the amps as well using two 2 x 250W modules from Class D Audio.
They sound great… providing a subtle (but deep) foundation for music – but also capable of room-shaking response on movies or bass-heavy music. I have two (bridgeable) channels left on the amp for a third sub, but I haven’t decided what to do for that one yet.