Here's some thoughts on how to amplify a banjo.
These are just ideas and my intention isn't to come off as a know-it-all, but just to toss out some concepts. If you are already getting a sound, you don't need my advice, but if you are having trouble, think about the following.

My theory is that if a person puts a pickup on a standard bluegrass style banjo it won't sound very good. One of the reasons is that the flat head style tone ring, and the acoustic sound most of the regular bluegrass type cats are going for, has this cool hollow open type sound. That's a really cool tone alright but really hard to capture with a pickup. In my experience, it's too complex a waveform to read and it confuses FX and samplers and stuff.
Also if a person plays a banjo with a pickup on it, with the same attack as they do playing acoustically in a bluegrass type band, it will sound like an icepick in your head. Also if the musician is unclear on the concept of how the EQ works good tone shall elude.

I've had success with Rob Bishline's wood rim banjo. The wooden rim produces a nice fundamental tone that can be read easily by electronic gear. It doesn't confuse electronics and modules because it gives such a fundamental tone. Also the slightly shorter scale allows you to run heavier strings which fatten up the tone. The wood rim has a fatter tone as well. This has worked very well for me. Rob worked with me on some ideas I had, and we came up with this set up. It allows me to play funky sounding stuff acoustically because of the thick tone.

So here's what works. Put a pickup on this type of banjo. What type of pick up you may ask? Well, it's best to try different ones because different banjos react differently to different pickups. I like the ones that mount in the bridge, but that's just me. Also make sure the wire from the pickup to the jack is shielded. If not your are going to get RF interference. Use a nice pre-amp that has sweep-able mid-range. The Treble Mid Bass thing doesn't dial in frequencies that well. A phase reversal switch is a good idea too. It's like a get out of jail free card. Experiment with your EQ and get good at it. You won't be able to set it and forget it as each night in a different room on a new system, the settings will change.
Here are some things to look for. The honky kind of icepick thing is somewhere around 2.5K or 3. The real high brittleness is around 6K. The woofy low end thing that sounds like your banjo is hooked up to an industrial shop vac tends to be around 200 or multiples of that 400 etc.
Find a chart on the net that has the notes that correspond with the frequencies. If a certain note "takes off" into feed back, you can find it on the EQ and attenuate it a couple of db. Or have your trusty sound man do that.
It's not cool to squish the heck out of everything and have a too many frequencies attenuated or boosted, that changes the phase of everything and makes the banjo sound funny, plus you can kind of fight yourself by squishing a bunch of stuff down and then turning up the gain and doing it all over again. You only should have one or two things cut or boosted probably. And usually only a couple db or one db. Maybe three. Don't get too drastic with the EQ. If you find you must, you probably have some other problem.
Learn to play to the pickup. If a person just blasts away like they would with the acoustic band, the sound will be thin and too crispy. A lighter touch is needed. And also since the electric stuff is an amplifier, remember that 1X2 = 2, but 2X2 = 4
Where 1 is the input, X 2 is the amplification and the sum 2 is the output.
If we raise the input one increment to 2, but we go up two increments on the output. So for an increment differential of 1, we get an output change of 2.

The point:
You get more out of slight changes in your volume electrically than you would acoustically. A very small dynamic change on your end comes across as a bigger change out of the system.

Also the acoustic sound is like a vector shooting away from you. It's very easy to be playing to banjo too loud to begin with.
You may have to move your hand in a little bit towards the neck. And play lighter. Listen and adjust. With different systems, things are going to change. This is good and not bad. Visualize a good tone and then close your eyes and move your hands around until it sounds good.

If you find yourself in a really loud situation, use in-ear monitors. It takes some getting used to but you can totally crank the banjo in your phones and play very lightly and not have to dig in. The sound guy can have your solos smoking out front. Wireless in-ear rigs are very inexpensive. The sound is consistent from night to night.

In extreme cases one could stuff the banjo, but I've never had to resort to that. Also if you know the material really well, it's ok to play where you can just barely hear the banjo. It's a good practice. I just saw an old Black Sabbath video and they had no monitors at all in the early days, and the same with the video I saw of David Bowie and the Spiders from Mars. So it's possible to play quite well in that format. For a long time, nobody had monitors. Decades.

Resist the urge to dig in and try to get more volume that way, your technique goes out the door and the sound gets all gnarly. And yes it will never sound like a loud acoustic banjo really, but hey...so what. My job is to re-contextualize all this stuff anyway so I'm off the hook. Also this never seemed to bother Sugarcane Harris!

Have fun.
Music is Good.

Danny Barnes 12.28.07