NO NO NOOOOOOO
be aware, the slave outputs aren´t an internal routing between two inputs.
the slave outputs feeds signal directly from the output, the insert point is just the same than 4,8 ohms output, the only reason you could use the slave output into line inputs is that mesa provides a voltage divider on the slave output that reduces the 20-30 volts of outputs into aproximately 2,5 volts ( a reduction with a factor of 10 )
For that reason I´m asking the hells reason to try to do such experiments