Hi!

When watching movies on this setup I found that without an active sub
basses were a little weak, so I pushed the bass to the max (+6dB IRC).

You shouldn't do that. If you want more bass, get a separately powered
subwoofer unit and use that.

The power amplifier in your Marantz unit probably isn't built to take
that kind of stress repeatedly, and the fact that the protection kicks
in is trying to tell you that, before something blows up permanently.
I doubt there is any lasting damage taking place, just the logic in
the unit is locking out normal operations until the protection circuit
is reset.

The service manual would probably tell you how to reset the
protection. It's likely that there is a hidden service menu or
keypress combination. Seriously, though, DO NOT just reset the unit
every time it trips out.

1/ Does anyone know the get out of protection mode procedure
for this model ?

No, but even if you find out, you should stop doing what you are
doing. The unit is trying to tell you that it can't keep up with what
you're asking it to do. You either need a beefier amplifier or a
dedicated and separately powered subwoofer.

2/ Any idea why this happens ?

Yes. You are exceeding the safe operating limits for the amplifier.
More bass output puts more demand on the circuitry, and it's probably
just too much for the unit to handle safely. So rather than
potentially ruining your speakers, it's just shutting down.

William