#1
The other day I picked up a Community CSX-70 PA cab (INFO: http://www.abra.co.uk/specs/CSX70.PDF) that a local Uni was chucking out, thinking that it could make a half decent bass cab as it's 4 ohms, 1500w and has a jack input. On closer inspection, seems the whole mid horn/tweeter section doesn't work and there are some structural issues (build quality looks pretty awful!!)

However according to the spec sheets these things are LOUD! so I was thinking of scrapping the tweeters and building a tuned vent in their place to maximise the cab's bass response. I'm not really sure on the best design to go for though, can anybody help?
A metal band?
Gear:
A Guitar with an LFR > Korg Pitchblack > Behringer EQ > Hardwire CM-2 Overdrive Boss SD-1 > Hardwire CR-7 Chorus>
Orange Tiny Terror >
LzR Engineering 212 cab

My other amp can run Crysis
#2
well, today I scrapped the crossover unit and the whole tweeter/mid unit, blocked the hole with some pine planks and generally went round sealing up all the little holes in the cab. Must have shaved some 15kg off the total weight, if not more, and of course it's now a 'closed' cab so the bass response will be... different.

Of course, I may add some tuning ports later if bass response is unsatisfactory, still tont have a clue about cab tuning though lol.

Oh and I picked up a SWR 750x today to run the cab
A metal band?
Gear:
A Guitar with an LFR > Korg Pitchblack > Behringer EQ > Hardwire CM-2 Overdrive Boss SD-1 > Hardwire CR-7 Chorus>
Orange Tiny Terror >
LzR Engineering 212 cab

My other amp can run Crysis
#3
I don't know about your particular speakers but old JBL product literature recommends ideal dimensions (below) for tuning to best bass response. Seeing as a frequency is a frequency no matter what generates it, I would guess that the following dimensions may be at least a good starting place for other speakers as well:

1.5 cubic feet interior volume w/4" diameter port for each of their 12" speakers* and 6 cubic feet interior volume w/5" diameter, 2" long port for each of their 15" speakers.

* This one sounded great loaded with a Jensen and an Eminence as well.

Note: I realize that your cab is already built but maybe the port sizes might be of some help.