"I won't argue your results, but with 35 years of repairing audio gear, I have not seen one yet that has that property. The in/out label on any gear is usually for that particular unit. The same exists on tape machines. There are input and output jacks there as well. The "output" jack on the tape should be activated when the tape is in play mode, thus is an output. "

That has not been my experience. In most amps the tape output is pretty much at the output of the input selector, or in the case of a Yamaha for example, form another separate selector. As such it can be backfed.

If it is buffered or even resistor isolated that will not work so well, but it seems the engineers decided that the low output impedance as worth it to keep the noise down. As such, not only can it be backfed, it can also be shorted by the wrong thing connected and cause no output. I got this DCD Pro CD player, commercial, like DJ model. I had it temporarily hooked up wrong because I couldn't really see in the back, it was going IN to the tape OUT. Well when the CD player was shut off it apparently shorts the outputs and that resulted in no sound from any source.

In another case I had a VHS HIFI, a Sanyo VCR-7200 that when turned off got non linear at the record INPUTS. It caused severe distortion unless it was turned on. It is easy to figure out why, the input stage was probably diode protected against overvoltage and when the power supply dropped, the shunting was to zero volts.

It is a piece of wire, the "current" can go both ways.

I didn't mention it before I think, but some of those units used a TC9164 or something as a selector chip and those did go bad. I had to change a bunch of them back when. That was how they were - the tape output was the only "input" that would work.

What I don't remember is if those chips fed a CMOS switch set or had it onboard. Like 4066s, or the cheaper one, 4016 ? Whatever.

I could probably find a print and find out but why don't you just to it ? Right from Google you got Electrotanya, and then there is hifiengine and hifimanuals. You need a membership to the latter two but they don't charge, they don't spam or any of that. I got nothing but good to say about those sites.

If you need the TC9164 they are out there. I have just Googled it and it seems they do have the actual switches onboard.

I know that amp is considered BFC by audiophiles but that series of amps is not bad. The circutry is fine, low distortion, all that. Anything the audiophools don't hear in it is their own problem. The only problem is they are underbuilt, for someone like me. I mean I will work it into 2.3 ohms n ****, they don't like that, they get too hot. That's why I don't use one, but for normal human beings the are OK.