trippy- I've played a ton of Ampeg 810s as back lines in all kinds of bands on the road. I actually think I know what "loud" and "deep" is, but I'll submit that's it's all relative. 810s get loud and deep. Good mids too. If it can't get deep enough for the stage I think there are some other issues happening. If 2 can't make it happen whoa....
I say go with an 810 if you can haul it (it's in your budget and a GB isn't). For the price of one GB you can get two 810s (used). No chance a GB 215 trumps 2 810s. Nope. Not happenin'.
Ampeg 810's don't go deep though.