ah yes the 4th order pand bass box its my fave well i dont know how to tune it because once its built its built the only way to tune it is by port lenth if you used a port that you insert into the box then what you do is you find a song that has a decent bass tone to it that is some what consistant and you slowly insert the port tell that tone gets the loudest and/or clearest then you cut the port to that lenth i have never done this but that is the only thing that i can think of
you can also download the nch tone generator and burn discs to the freq. that you want to continuosly play while you trying to tune your box. lost the site, but if you google nch tone generator, I'm sure you can find it, plus it should be in previous posts of a couple weeks ago, that's how I found it.
A bandpass box needs to be built for a specific driver using its TS parameters to find the correct volumes for the front and rear chambers and the tuning frequency of the port to perform at its best. Why bother with the difficulty of bandpass when there are other easier design that won't sound like **** if you build them wrong?
On the other hand as I've got shares in PVC tube,MDF and carpenters glue and the fact those bandpass boxes are just so cute checkout this site