I prefer to tune in the 'teens, lowest I've done is at 11 Hz, and I liked the sound of that box a lot, except that there was too much airspace, next time, I'd force tune it lower, or, if you want to lower the tuning, stuff part of the port with polyfill, and keep it in place with chicken wire or something similar, or put an aperiodic membrane in the port to "fool" the port into acting like it's longer... even better, put 2 aperiodic membranes in the port, with a slightly enlarged section between them to resonate and lower the tuning even more...

just what I've seen and experimented with.

That's my oppinion, it's worth what you paid for it.