Ok, I'm game. 
None of this relates to the original question.
A slave amplifies a line level signal to drive a speaker system. A direct box will provide a line level signal to drive the slave. Any adjustments to the tone stack of the Sorado will affect the output signal of the Sorado, and thus affect the signal applied to the direct box, and thus will affect the signal applied to the slave. Therefore it is not necessary to tap into the preamp circuit on the Sorado.
A direct box is just another means of getting a signal, and miking the amp through the speaker is not the only means of getting that signal affected by the preamp controls and just making it louder.
Ok, so what did I misunderstand?
Maybe I'm just naive, but I've never seen a direct box that takes a signal from the output stage. My only DI experience has been with units that convert the unbalanced high z signal from my bass to a low z balanced signal that makes the console needs to see, but indeed taking this signal
before the tone stack. I even keep a Whirlwind DI in my gig bag for the rare occurrence when I inadvertently "bring a knife to a gun fight" and need just a
little PA support.
This also should work for providing an input signal the slave, but you would lose the tone shaping of the primary amp's tone stack.
I think I would prefer the fat signal coming through my cabinets and into a SM57 over a post output, pre speaker cab DI signal for situations such as outside when I become largely PA dependent.
But I digress, the thing I like most about this and other music forums is learning new things. Now I know that at least one company makes an output capable DI, but I'm not exactly sure why;
...other than this thread starting dilemma of course.
