I don't think I have the answer necessarily. One particular thing that comes to mind:You have the answer before even asking, of course it is fine if it is seeing the appropriate load. You wont break your amp.
Terrible interface like you say, but it works.I found it somewhat excruciating to use, with poor user interface, and it doesn't even have native support for potentiometers!
Just put in a post phase inverter master volume and tweak with that. But that's my solution.I've seen that John H attenuator before, but I'd much prefer to make one continuously adjustable, with a rheostat, not just with discrete switches for discrete attenuation levels. I find with the attenuators I own, I do a significant amount of tweaking of the dial to get it just right. I'm not sure discrete switches will allow me that flexibility and the fine-grained control I want.