Well, there is no worry really. I was just goofing around last night with the MUTE output when I realize I could try to use it for silent recording. Then again, if the signal for the slave is from the speaker's signal it would mean that it's post power section? From the behavior of the MUTE output I did not think there was any power distortion. I could be wrong. I definitely don't quite understand the OUTPUT/SOLO/MUTE/SLAVE section of the amp.
Now the more I think about it and the more it make sense that the MUTE out is simply related to the OUTPUT/SOLO circuitry. I remember it's on the push/pull SOLO knob, right. The fact that it's placed closed to the SLAVE out at the rear is only logical but does not use any of the slave components I would say. But at this point it's only speculation. This approach is also used in the Lonestar I was told and maybe on other models so that has probably been discussed elsewhere...
Would that preamp signal be a decent signal to do simple direct recording through speaker sims and other effects?