I've been down this route recently with one of my bands, only done a couple of gigs so far but FWIW here's a summary of my experiences:
Started out using Shure SE535 (which I already had) - these have a very flat response which I found wasn't ideal so I got some KZ ZS10s (as suggested in one of the existing IEM threads) and I'm very impressed for the cost, the bass response is significantly stronger.
Preamp - I use an HX Stomp direct to the desk, personally I didn't find the amp/cab models worked that well in a live mix, so instead I'm using a compressor block and the ZeroAmp Bass DI. I don't normally use compression, but without any natural amp/cab compression it's helpful.
I'm also considering moving away from the HX stomp (at least live, it's a handy thing for recording/practice) since I don't use 99% of the features, probably will try a compressor and preamp pedal (Sansamp or maybe Origin Effects Bass Rig).