It's good to know that you're concerned with the spectral balance of your system. Many believe that a system can never be "too bassy," but music is a blend of many elements. Your only concern will be matching the main speaker system with the subs. You can jam in as many subs as you want, and I agree with having plenty of headroom to allow for solid, body-blow bass, but not at the expense of the overall balance of the system.
The biggest obstacle will be getting mids and tweeters that will keep up. As I mentioned earlier, the tiny voice coils on small drivers will not handle the heat that a subwoofer can endure. While 6x9 speakers will go a long way, my bet is that you cook the tweeters. Whenever I build a system that's meant to play loud, I know that I'll be forced to sacrifice a bit of sonic purity to achieve sound pressure. I'll try to locate the highest efficiency, highest power handling mids and tweeters I can find, and if that still isn't enough, then I need to start looking at multiple drivers. Clustering two mids and three tweeters in a tight array will provide lots of extra output while remaining relatively reliable; and by keeping things as close together as possible, I minimize frequency dependent shift, as much as is reasonable. At higher volume levels, nobody can hear the sonic problems anyway.