sub question

The tape out has no volume control in the signal path, so you wouldn't be able to control the output to the sub.

You could do this, but you would have to adjust the sub's volume from the sub itself to match the main speakers. I guess, if you happen to listen to the exact same volume level and setting each time, you would be okay. Otherwise, you're going to spend a lot of time readjusting the sub's volume.

Yes you can ,but like Mofi said ,it will be real PITA to adjust volume level. Your other [and recommended] option is to find a sub with hi-level [speaker level] inputs.

You either need to run the speaker outs through the sub's crossover or get an REL that will tap into the speaker outputs from the brio without interfering with the signal to the speakers.