I think what Standby really does is prevent hi voltage to go through any tube, but still allows them to warm-up at the same time.
If you have a solid state power section, then fine, but as I said, I wouldn't let you borrow my amp
Even if it was safe, doesn't work with the XXX though, tried already, but with a cab connected for silent recording in my Room.
No, and I don't think it will ever.
You can buy cheap resistive loads for that, or DIY yourself one. It is worth it.
In the long run, you'll blow your amp is you do like Arvoitus says, even if it supposedly works on Engl, that, I don't know.
AND remember than the usual way to Master/Slave amps is NOT by the effect loop, but it is better usually done with the Line out.
I'm not an amp tech and I can't really tell you the difference between Fx Send and Line Out, but I know that you usually slave an amp through it's line out. You can either put a cab or a load on the amp that sends it's signal to the Slave, but you NEED a cab on your slave amp.