Bobcoss,
you are right, I actually mostly don´t use the amp modeling because my sound reference is EVH. These kind of sound you only get with real power tube distortion and to be honest, you need also speaker distortion, but this is another issue. So I treat the 6505 and the load box as one stomp box. The load box out is of course the line out.I can hear nicely the difference of cranking the master and cranking the pre gain, also in the 6505. To have an idea, pre gain is on 1 and post gain on 10.
The HXS send I tried both with line and instrument level. Also the HXS Return I tried with both. The feedback is always there.
Interesting is the method to run guitar in the amp, loadbox into return of the HXS and assign the loop to return. This works without noise but for some reason all blocks before the loop do not work....
However, there are posts about normal stomp boxes which create feedback loops in the Helix. I never tried to run my slaved amps in processor loops before, but it should really be possible.