I presume the "audio out" is "attached" to whatever is actually playing. So it spits out either Audio 1 or Audio 2.
It (should be) line out. So yes, you can run that out to another pre-amp(or amp).
If you are feeding a pre-amp into Audio 1 or Audio 2...then you are making a volume adjustment that the "Audio out" is simply relaying to an amp.(granted I haven't re-read your post, cause I was confused on what you did...and didn't want to comment till I knew ALL of the equipment involved).
Now, you could also be running this audio out to a pre-amp, then onto an amp(really won't make any difference which way you've done this, as long as a pre-amp is involved somewhere)
Basically, what you've done is (sorta) create your own 2nd zone...
I did just re-read the post and I still can't make heads or tails out of what you've done...
To use that "audio out" to an amp, you must have the pre-amp somewhere.
This scenario could work(although I am SURE it isn't notated in the manual ANYWHERE.
You are playing a source. You feed the source out the "audio out" which feed the pre-amp. The pre-amp runs the tube amp. "Somehow" the receiver isn't "frying itself" because you have nothing connected to the front channel(frying itself is a harsh term).