Wow.... lots of coincidences here; When I read the original post, I was thinking to myself "this guy needs to get the Sloane power amp construction book and check out esp" (site that Helix mentioned - in addition to what he said, I also recommend looking at his Linkwitz transform, sub-woofer controller & sound impairment monitor projects).
AND it just so happens that I got the pcb's for both the 300w amp and the active x-over he also mentions just last week! I worked out that for me to buy all the parts in the b.o.m. for the amp, plus the power supply, plus some stuff to homebuild a 2U rack case - it will cost roughly the Aussie equivalent of $U.S.150. Thats pretty sweet for an amp that size!
I have to wonder why you want a sub amp that develops 500W? You must have one hell of a system to add it to!! That other site that Helix noted - Aussie amps I think (?) has I believe a 1Kw MOSFET power amp kit
here. If you build something like that and you get the sub down to an f3 of, say, 10Hz (heh, heh, heh) - you should be able to use it to experiment with sub-sonic soundwave weapons technology, or at least use it to demolish houses with vibrations!!
While I'm on the fun and crazy theme, if you do ever build that 500W class-A valve amp, you can you can turn it on it's side and use the heatsink to barbecue whole cow carcasses.......