Hi again GJC,
2 x 600w class D amps would be a good choice for driving a XXX. One would stuggle and I expect, as most under-driven subs do, that it would yield a less flat frequency response since there is unsufficient power to control the cone. So make sure you're looking at 1000w minimum, a single XXX handles 1600w RMS nominal and 2500w RMS peak.
I can offer my advice on the various types of material that I've used during my time spent building boxes:
Plywood: Good for small enclosures when using at least 10mm thick ply not really suited to larger enclosures unless double or triple walled. A good point of the very thin plywood is that it can be used as a veneer after sanding smooth then stained and varnished. Done this on a few boxes in the past and was always pleased the results for very little cost and its extremely easy to do.
Chipboard: Don't like this myself, only used it once and was never pleased with the results. Its not very nice to work with either since its a brittle wood. It is however very cheap and available finished in melomime (the stick on plastic veneer) so that will save you time on finishing.
MDF: This is the daddy for cheap and very solid cabinets. Its particularly suited to subwoofer cabinets because of the thicknesses available and the solid structure of the MDF. Most people double wall on larger cabinets but either way it has good non resonant properties and is very easy to work with - sanding, cutting, routering are all very easy with MDF. Always wear a good mask when working with it since its deadly stuff for the lungs - in case you didn't know it contains formaldahyde and is banned in the US!
Solid woods (Pine, Oak etc.): Very nice if you can afford it, hence I have only limited experience with this building small enclosures. It is of course the best material to use if cost is no object but larger pieces cost and if you need a particularly large piece joints are required.
Plastics: No experience whatsoever with these but would be interesting you have the tools to work them.
If your interested in designing a box then try the following:
http://www.linearteam.dk/default.aspx?pageid=winisdpro
http://home20.inet.tele.dk/kou/ubmodel.html
There are many great programs to help you model a particular driver in your choosen box volume. These are two of the more popular ones available. They give you a 'rough' guide to how each will perform in a particular situation, don't forget that room gain is not factored into these models and you have to work that out yourself depending on your room dimensions. Usually its around 3-6dB as a very rough guide.
Let us all know once you've started out