I have been using preamps into poweramps for close to 20 years. When you are using any type of pedalboard/multiFX that includes any sort of preamp or amp modeling, the worst thing to do is to plug it into the input of an amp. You then have 2 tone circuits battling each other. Prior to getting my HD500X, I used a Digitech 2120 rack mount preamp/digitalFX rig, usually direct to the PA. If I did use an onstage amp, I would run the Digitech, fully stereo, into a Mackie m1200 poweramp connected to a stereo split 4x12 cabinet.
Another option I have used extensively is running the preamp (HD500X, Digitech 2120) into a pair of powered monitors. Set the EQ flat. This route gives you complete tonal control of your sound since you can program your patches accounting for the horns in the monitors, which in turn allows you to run the second set of outputs straight to the FOH with little to no EQ adjustment needed at the board. You will be giving the soundman exactly the sound you want portrayed to the audience. I usually then place the set of monitors in front of me so I have absolutely no issues hearing myself while also creating almost zero stage volume. It lets the soundman have complete control of your levels in the mix.
