I completely agree. 4CM with the HD500(x) should be as simple as it is with the M13. It isn't. You get a 4dB drop at the effects send if you have the loop configured at Stomp level, and that's problematic because you don't want a lower signal going to your amp. Fortunately, there are several good ways to fix this that don't involve taking up an additional effects block:
1. Input configuration: Input 1: Guitar, Input 2: Aux (In this scenario, we're not going to use both parallel signal paths. You really don't need to.)
2. Make sure the amp model is disabled and in the "Pre" position.
3. Set the left mixer channel pan to center.
4. If the FX Loop level is at Stomp, you will need to put the FX loop AFTER the mixer and set the level of the left channel to +4dB.
5. If the FX Loop level is at Line, you can put it before or after the mixer but there will be a +8dB boost at the send. Set the loop's send to -8dB.
Either of these approaches work, but I prefer the loop level at Line and decreasing the send level because then you aren't adding noise by boosting something.
Once you get past this problem, 4CM isn't that bad. But the fact that I had to troubleshoot this with signal generators and meters means that it isn't intuitive at all. And it certainly isn't as simple as the M13.