I now have three Sadowskys... no need to tell me how great they are.
Currently, I'm using the Reiner Amplification all-tube preamp designed and built by Jonathan Reiner, a.k.a. "JGR" here at TalkBAss. I've no affiliation with him other than as customer. I love this pre. In short, it's the Alembic preamp (F2B or F1X) on steroids: two boost/cut mid controls, with gain and master volume so it's easy to dial in the exact amount of overdrive you want... or none at all. The downside is that Jonathan is a one-man operation, so the boutique quality comes with boutique price: it's close to but not quite in your price range.
Prior to that I used SansAmp preamps: specifically, the RBI and RPM run in parallel. I recommend these too: they sound great (difficult if not impossible to tell they are solid state, not tube), and are less expensive than many other preamps. Each is around $330 from MusiciansFriend, easily within your budget.
