I use the 4cm with a bogner atma. Took awhile to configure, but I've got it running pretty good. Here's what I've learned.
1. I only use a humbuster cable between the output 1 and the return on my amp. If you use a humbuster cable from output 2 to the input amp, there will still be a little bit of hum. To completely eliminate the hum/ground loop, I use a high quality iso transformer with ordinary TS cables (lehle p split). I only use the iso out on the lehle. This breaks the ground connection and eliminates all hum.
2. use short cables
3. I set my output 2 level knob on the front maxed for unity gain, but use output level 1 as a master volume. That way I can just set the relative channel volumes on my bogner and leave them. Then if I need to turn up the whole amp, I use the output 1 level knob.
4. additional hiss cannot be avoided in the 4cm if you use a distortion channel on your tube amp. I wrestled with this a lot this summer and after much research, realized that its unavoidable. In the 4cm, the axe goes before your amp input. Putting any digital device before an amp's input introduces some noise. When your amp is on the distortion channel, this noise is amplified because of the gain. People who use clean sounds, or overdrive will not notice the introduced noise, but its noticeable on the higher gain sounds. Having said all this, the additional noise is not that bad. I did an experiment where I put a TC electronic Voicelive 3 (has an amp modeller) before my bogner amp's input on a distortion channel. The hiss introduced was brutal, and significantly worse than the axe. It was then that I really realized that the axe does introduce very little hiss when in front of a higher gain channel, and that it's unavoidable. Increasing the pad setting on output 2 helps reduce the hiss even further. With a noise gate on, it completely goes away.
5. Bypassing the real tube preamp in the 4cm, and using the amp models in the 4cm is great, but the tone is shaped quite a bit by your power amp and cab.
6. It'll take several hours to get your setup sounding right. Don't give up. The hum can be avoided, and the hiss minimized. An iso transformer is critical though!