Of course the USB is a digital signal and the buffer is an analog device, so any use of the buffer will have to be after D/A conversion.

Even if one were to manage to successfully route a digital signal through this buffer, the data would not have been altered.

One would appear to be left with two options: replacing the 'pre-out/main-in' jumpers with the buffer or using the tape out/in loop. The first option means all signals are affected all the time, the second option means that one could listen to a source directly or routed via the buffer.

Personally I regard such 'tweaks' as dubious.