Unless the source is a turntable connected to the AI 2C phono input, most preamp circuits simply loop the line-level input signal to the tape output. Of course, there are exceptions to using active buffers, but I doubt that's the case with AI 2C.
Connect a source to the line level input. If there is still a signal at the tape output even when the AI 2C is powered off, this will confirm that there is no active circuit between the line level input and the tape output.