The Amateur 3 class would be the only one with no limit on sq" for subs, and yes, they will still be a music only class.
5000 watts is a bit much for any of the Amateur classes. Mike has a point, you can pick up some of these for cheap on these forums and some of the other ones too. But, I have yet to see anyone come out and attempt to compete in any Amateur classes with any amp that they couldn't buy at Best Buy or their local retailer. A DD M3 is a great amp, but if you think about it, what rookie is really going to have a clue about the potential of such an amp? The idea of Amateur classes is to keep it more of a level playing field for those that don't know a lot about SPL but want to learn. They have the chance to learn and do some work on their systems playing music with a reasonable amount of power, then if they feel they know enough to take it up a notch, then they can invest in more power, batteries, wiring, HO Alts, etc and jump up to the Stock classes. If there was a limit of 5K in Amateur you would need the 3 batts at least, plus a lot of 0 ga wire and a beefed up Alt. Putting that much of an investment into a system really wouldn't be something an Amateur would be willing to do to be competitive. Gotta keep in mind that Amateurs for the most part are new to any kind of SPL comp and are just coming out with their daily system to see what it hits. Some get surprised how loud they are and want more. If you have a power limit thats too high it'll scare away potential SPL competitors. Not everyone gets hooked on SPL after their 1st show, so you gotta keep it fair so they will come back out for a 2nd show.