Auto powersave - How to retrofit on older KRK Rokit active monitor?

[TinkeringSteve]

Helloooo,

an understandably sometimes hated feature of newer models of the KRK Rokit active studio monitor speakers is the auto-power-off when there is no input signal for 30min or so.
Here, someone modded his newer speaker to be able to (de-)activate it at will:
https://www.eevblog.com/forum/repair/krk-rokit-5-g3-how-to-permanently-disable-the-auto-power-off-feature/

Now, I won't  have it that easy, because my 2nd gen or so model does not have that feature, and I can't just bridge some IC

How would you design a feature like that, to retrofit an older specimen with it?
My hobbyist tinkerer's first thoughts:

• some power will always be drawn, to be able to auto-turn-ON also, by detecting a signal
• unless..., then again, I wouldn't mind if it cannot do that and I had to press a button to re-enable
• the latter would allow to save even more power, as no signal monitoring needs to go on, when it was switched off
• i.e. in that case, a relay may cut mains power from the 3-contact mains jack - probably also the easiest point to intercept
• do even small relays, and DC ones, hum, due to whatever miniscule instability in the setup? Perhaps using a mechanically latching relay is advisable? Although I reckon they may have fewer guaranteed cycles than non-latching ones
• how/where to tap the signal in the chain for detection, without introducing noise?
• is it too wild to assume the power supply in such a thing will have enough headroom to allow for the extra current draw of a relay? I think these still have analog amplifiers that draw a lot of current all the time, hence it seeming advisable to add this feature
• signal dection/action could go like... comparator with certain threshold -> RC (dis-)charge for timing -> another comparator for disabling the power? (I normally just throw a cheap MCU at things, but it might add noise here )