Yo Myriad! First, I disagree with you regarding Avalon, but I agree the U5/AD2022 wouldn't be my first choice for heavy rock (except as a bass DI). For a string quartet, it might be.
That being said, you started talking about how to spend $320, and these guys are trying to sell you a U87! If you find one for $320, let me know. Looking at your cabinet, frankly it doesn't suck. You have chosen respected mics that are very good at their price point. Now I could recommend a ton of expensive mics that would expand your cabinet's capabilities. but so what? That's not what you asked for. Here's the best stuff I can think of-
Oktava MK319- Any female vocalists you have will kiss you for it
Oktava MC012 matched pair from The Sound Room- much better rock overheads than the NT5's, IMHO. weirdly- for some folks they are pretty good vocal mics, but probably not for hard rock.
Sennheiser MD421- what the SM57 wishes it was
AKG C2000B- no shit- stick this little bugger in front of a cab and it will surprise the hell out of you. A pair make great overheads, at least as good as the MC012's, and they work very well on some vocalists, as long as they aren't big time screamers. One of the most underated cheap condensers.
Any of those mics fits in your budget or is damned close, and will do something no mic you currently have will do. Just a note on the MC012's- I'm talking about the Russian ones. The Chinese ones are OK mics, but they are almost the same as your NT5's. And I'm dead serious about the C2000B being a kickass cab mic. Good luck-Richie