It's a never ending problem isn't it?
I'm currently going through the mill of finding a new pickup. Had a lot of advice from sound engineers, bass players and producers. Also some jazz guitarists who've had similar issues with hollow body guitars and feedback etc.
Reached a conclusion for my own circumstance, might not be suitable to anyone else..
I do a lot of loud volume gigs, as well as quieter ones. Personally, I need to amplify my acoustic sound as accurately as possible. For pizz, and for bowing. I can't see the point in spending a moderate amount of money on something which you need to effect afterwards in order to re-create an acoustic sound. If you're really after it, you're probably going to have to pay up...
A lot of the really top bassists out there seem to blend 2 signals -one mic, one pickup. check out Dave Holland, john pattituci, ron carter, charlie haden, christian mcbride and so on.. A common choice seems to be the realist, blended with and mic by AMT -SP25B (http://www.gollihurmusic.com/product/1472-AMT_UPRIGHT_BASS_MICROPHONE_SYSTEMS_SP25B_S25B.html) or something similar. You send the mic straight to the PA, then the pickup to your amp which then also goes to the PA. That way, you can get all your clarity from the pickup, and your wood/umph from the mic. Unfortunately, that really is a big bill to pay, and you need a sound guy who actually knows what he's doing.
What I'm going to try is getting this Schertler stat select pickup, which seems to be rated pretty well, and put it through an EQ pedal -I've found a list of the frequencies you should boost/cut out for double bass when you're recording, and for live to avoid feedback. Another big influence in feedback is the fact that the bass is effectively it's own amplifier anyway -it's that big body that's causing the frequencies to behave like that, so I'm looking into doing what guitarists do, and buying to plugs for the F-holes for those really loud gigs. That [i]should[/i] sort it out.
I'll let you know what happens..
xx