I had a look at this around 10 years ago. I am pretty sure that what I will write was correct then, I suspect it is still true. If not, I would be happy to be corrected.

The (orange RCA) Remote In (and Remote Out) are not normal IR Inputs (essentially connected to an IR LED, possibly in series with a resistor) According to the manuals, it is intended to connect to other Maranz equipment, i.e. it is a propitiatory interface. Everything is not lost though: I looked at schematics, and looked at the signal with an oscilloscope, and it turned out that it corresponded to a normal IR signal with the logical levels reversed. Differently put, with 0 volts, the IR LED lights, when the signal is "hi" (5V?), the LED is dark. Connecting this input to a normal IR connection block (0: dark, hi: light) still may work, due to the modulated nature of the IR signals -- Maranz, at least in the past, is using the RC5, RC5x, or RC6 protocol, al having 36 kHz modulation. Differently put, a demodulating receiver detects a modulated signal, it does not really distinguish between constant on and constant off. (However, it may still be a bad idea, since the circuits are likely not desiged for LED on 99% of the time.) So this may explain why it worked with only the Maranz connected, but not with more devices connected.

SO, if I am right, you only need a signal inverter. (topic for a separate post...)