What is it? I just came across one. It's a short barrel with male 3-pin tuchel on 1 end, female tuchel on the other, & an on/off switch not quite in the center.
Check the input/output connections just to make sure they're still intact. If that's not it, time to have someone take a look inside to see what failed.
Bring the tape recorder outputs to the M35 tape ins. You can find the M35 manual online in several places. Download and read it thoroughly and all head scratching will be eliminated.
I assume that you just raise the volume of the amp at the head. If the dummy headphone plug turns the cab off, it's off. Volume boost won't change that. What does the amp's owner's manual say?
Use the preamp out to your interface. I doubt the phones out would sound all that good even if it did work. In addition, plugging watts into an input expecting line level might be detrimental to the interface.
1. Right click and drag mouse between the 2 cues to highlight area. 2. Press control & c together. 3. click on where you want to paste. 4. Press control & v together.