It's honestly slim pickin's in the $500 range for single-channel pres/channel strips - and I think mostly a waste of good money, IMO. There is the Grace 101, which is transparent, very clean, with a bit of a glassy sound. If you do a lot of acoustic stuff, I'd recommend the Rane MS 1b. Runs about $150. You could get two of them for $300. Don't be fooled by the price. I don't even consider the MS 1b to be a "budget pre". They just happen to be inexpensive. I think they're easily as good as the RNP, and probably better for instruments like acoustic gtrs and banjos. The MS 1b has a lot of similar qualities to the John Hardy M-1 - which is a top, high-end clean pre.
That'd leave you $700 to get something else. You might be able to find a used A Designs MP-1 tube preamp for that price. You can also get a new Sebatron vmp-1000 in the $700 range. That's a real, high-voltage tube preamp.
Though it sounds like you could use some better mics. If you record a lot of acoustic instruments, choirs, drum OH's in stereo, I'd recommend the Peluso CEMC6's - SDC mics in the $600 range for a pair. They also have optional-pattern capsules.
A pair of CEMC6's and a pair of MS 1b's would give you some pretty serious tools for stereo recording, and be a really solid way to spend a grand for what you're doing.