[Moonfly]

What is your budget? If you can accommodate a 15" driver it would be rude not to, but its not an absolute must. As for the design you want to use, that really down to what you want to achieve, have a read of this:

http://www.danmarx.org/audioinnovation/theories.html

Then choose the design you feel would best suit you. Tell us what you really want to use the sub for (music/movies) and if you just want a nice sound or you want real low chest thumping bass, and we can advise on if your chosen design will best suit you or not.

I could easily just tell you what I think you should build, but this way you'll learn as you go :T

 

[Moonfly]

Your going to have your work cut out, especially if you are in the UK as I suspect you maybe. The Monolith uses the peerless 12" and gets just about the best from it, with the cab size and tune being pretty much optimal. BK did try bettering it themselves but the test build didnt deliver what they hoped and it was scrapped off. You might be able to tweak the design and gain some small improvements but nothing major.

If your based in the US, then there are more options available as components (mainly the drivers) work out cheaper due to not having import costs added. We're pretty restricted here in the UK when it comes to good affordable 12" drivers.

Again this advice is location dependant a little, but if you can use a 15" driver, you can build something like a Monolith sized sub that will give a comfortable performance leap.

 

[Moonfly]

Well this wont be much help, but in the UK, for good quality affordable 12" drivers, the peerless and the Eminence Lab 12 are about it AFAIK. As for 15" drivers, well I got mine imported from the US :scratchhead:

I'll try have a look about and see what I can find that may be of use.

As for the argument of sealed vs ported, it really depends on what results your after. Have a quick read of this and see what you come up with, then we can take it from there.

http://www.danmarx.org/audioinnovation/theories.html

There are euro distributors of drivers I can point you too ( the dayton link is broken), but you best deciding what you want to build first. I would go with a 15" driver if you can though, just my preference.