Can't the redbox take a speaker out? Like you said, neither substitutes for a load. I have the Allan Holdsworth juice extractor that will take speaker outs, and it can "be" the load too, like a soak unit. But then it has parametric EQ and some preset switchable cab simulations. You can find them sort of cheap now. But if you need two for stereo it would get expensive. I like the switchable sounds though. One of my beefs with a simulator is that when you get it to where you like it for overdrive, it usually "nasals" up the clean. If you set it for a rich clean sound then the fizzle comes back a little on the distortion. And a preset "4x12" one is usually somewhere in between. An actual speaker and mic has more flexibility between these sounds because the prominent frequencies are fighting for the same "space" out your speaker, so in overdrive, the fatness will "out do" the fizzle, but for clean, those sparkly highs get to come out. A PA cab with a horn or a bi/tri amped system gives a flat response, and it's up to the simulator to get it right. With the Holdsworth you can switch between the two sims, switch to flat, and bypass the parametric.
Direct is okay so long as you're not so "heady" about your tone. I mean we all know its not the real thing but is it "good enough?" All I'm saying with my huge rant is that if you don't like the sound of the Ultra G don't give up on the idea.