Pretty simple fix for that. Take one dried habanero, three or four Japanese or Thai chiles, a dried Chipotle and maybe a nice dried Anaheim chile or other milder chile and run them through an extra coffee grinder like Braun or Krups make. Grind until dust. Be careful when taking the top off and don't breathe the dust and if you get it on your hands, wash immediately. Add to rub. You'll probably have to do it in two or three batches due to the volume of the dried peppers.
It's kind of hard to make rub too spicy because the cooking tends to make it milder. Unless you just go way overboard on the ratio of chiles to base, you should be fine. And don't substitute the ground chiles for normal commercial chili powder, if you're already using that. Keep the commercial powder in there as well.
To me, things that give a good rub "zip" aren't just the hot things. Cumin and coriander are important flavor components (at least to me). I prefer to toast cumin and coriander seeds first and then grind them as well. Much much better than old stale jarred cumin and coriander powders. I make my rub in a similar fashion that I make an authentic Indian curry powder. I use as many freshly toasted spices as possible. I even use my countertop convection oven/toaster to dehydrate habaneros, since they aren't all that common. Otherwise, I just buy the packaged dried chiles.