The Arduino should be operating when the preamplifier is one and when it is in standby/sleep mode, so preferably power consumption should be low.
Is the preamp mains or battery powered? If it’s mains then power consumptions is not that much of a constraint and isn’t worth worrying about. If it is battery, what voltage and capacity and how long does it currently last? How much life are you willing to give up to power your additional circuitry?
Power management is a significant undertaking in and of itself. It’s not worth it for something mains powered. Focus on the important problems, like learning how to manage the controller’s I/O and not introducing extra digital noise into the system, first.
It needs a total of 4 digital/binary inputs and 12 digital/binary outputs
An Uno can easily handle that. The 6 analog inputs can be used as digital, so there’s effectively 18 pins to use (2-13, A0-A5). If you need more I/O and it doesn’t need to be fast, you can use shift registers or port expanders so you don’t have to splurge on something more expensive like a Mega.
On top of that I want a front panel display that can show the used input channel and volume, for example “ch1 -34dB”, so preferably 9 digits or so. It should preferably have color text (orange or red or so) on a black background. One line of text is sufficient.
Adafruit has RGB backlight negative LCD displays. It looks like it uses a lot of pins, but you can get serial backpacks to solder onto the back of them that will let you use SPI, I2C, or UART to control them and save many pins.
Next to or integrated into the display I want an IR sensor that can be use to control the pre-amplifier with a remote control (input channel, volume control etc.). No mechanical controls (knops) are needed, IR control only.
Most IR remotes are probably NEC protocol with 38 kHz carrier frequency. You can easily find 38 kHz IR receivers, and the IR library uses interrupts to decode the serial stream so it’s not that CPU intensive to receive data.
For automatic input channel detection, however, there will be some electronics (opamps etc.) OUTSIDE the Arduino to detect if there is a input signal signal on a particular channel. But this status per channel will be sent to the Arduino as a single bit (true/false) per channel: 1 or true if there currently is a spdif signal on the channel, 0 or false if there is no spdif signal.
I think that’ll be fairly easy. It looks like S/PDIF uses a type of Manchester encoding, so there will be constant transitions on the line when data is flowing. Use a Schmitt Trigger or optocoupler to buffer the signal and use it as the input to a retriggerable one-shot, like a 555 timer. Set the on time equal to the length of a few periods of the digital signal. As long as the signal is active, the incessant switching will cause the one-shot to be continually retriggered and held active. When the signal goes away, the timer is allowed to expire and go back to the inactive level.