"Before pre-amp" compressors (not studio compressors): I have no experience with them, but I keep reading about them and how they can add a bit of sustain on open notes and even out the attack for a more "processed" sound.
I know there is a number of drawbacks but I was wondering if anyone actually used those here or if most of you think it's a myth or something only some arena-touring technicians were putting in the depths of those giant fridge-size racks for the band.
For me the reason I don't have one right now is just because I don't need one. I have plenty of sustain while still having a very tight, responsive gate setting.
My general rule of thumb is that if you're going to use a compressor you should also use an EQ, since you're going to be emphasizing the frequencies you don't like just as much as the ones you do like. If it's before the pre-amp you may be able to get the tone you want just using your amp's EQ options but I prefer something more surgical to really carve out the nasty lower mid peaks and the nasally/flat higher mid frequencies.
I'd also say to get one with lots of controls, don't just get a Dyna Comp and be done with it. The MXR M87 Bass Compressor has lots of controls and visual display of your compression levels too so you get a better idea of what you're actually doing.