Yea, but check for inserted jack port can be relayed to second jack port also - then no need to using dumb. If, at the same time, we incorporate another check only on second jack port (for example, "probing" other portion of jack plug) and use it to disable/enable amplifier, we got 0 additional effort solution. That electronic parts i still can't remember name for are very small, if used for such a low voltage like <5v.

I really, really can't remember it's name... Is the same thing that start water cooling pumps connected to 230V when computer starts, in desktops sets. Of course ones for 230V are MUCH bigger, but as i said max 5v oneis so small that can be put inside cover with ease.

Of course doing everything like this is little hassle, but after that You don't even need manual switch to turn amplifier on/off. Less cutting on Your cover, less elements that can possibly break due to mechanical moving parts