That is the point of having two ABY boxes.
When you want to use the 360+ as the only preamp you use one ABY to redirect your pedalboard's output from the preamp in on the amp to the power amp in.
When you want to use the built-in preamp (amp) you use one ABY (reversed I guess) to send your pedalboard's output into the instrument in on the amp, and the second ABY, the one described in the scenario above, basically now acts as simply an instrument cable in the effects loop which basically does nothing.
If you have a simple cable in the effects loop, you are still using the built-in preamp.
Another user posted how he does it with his Bassman. I would have to look back at my recent posts to figure out who.
When you want to use the 360+ as the only preamp you use one ABY to redirect your pedalboard's output from the preamp in on the amp to the power amp in.
When you want to use the built-in preamp (amp) you use one ABY (reversed I guess) to send your pedalboard's output into the instrument in on the amp, and the second ABY, the one described in the scenario above, basically now acts as simply an instrument cable in the effects loop which basically does nothing.
If you have a simple cable in the effects loop, you are still using the built-in preamp.
Another user posted how he does it with his Bassman. I would have to look back at my recent posts to figure out who.