Yeah 3000 cubic feet is really pushing it for a single PB10. I'd want dual PB10's at minimum in that room. Honestly, though, at a budget of just $500 you probably won't find anything that's a huge upgrade over the PB10. The Hsu VTF-2 MK3 might have a little more output in maximum output mode, but then you'll lose extension. The PB12-NSD is a good upgrade but I'd be surprised if the 3-4dB increase in output it yields over the PB10-NSD were 100% satisfying to you, being that you seem to be driving the PB10 to its limits all the time. I'd guess the A3-300 performs similarly to the PB12-NSD, though I don't know...good luck finding someone who's heard both, though.
Anyway, your best bet is to maybe add one of those other subs to the PB10. Your post wasn't clear as to whether you're planning to add a second sub or replace the PB10 with one of the aforementioned subs. Again, if you're planning on swapping the PB10 for one of those you mentioned, I'm sure you might get a modest increase in SPL, but I'd doubt you'll have your socks blown off. Adding say a PB12-NSD to your existing PB10 might yield better results. Or, if you want to sell the PB10 and then look at subs in more of the $650-1000 range, that would get you a sub that should offer you awesome performance. Just my two cents. Good luck!