My Denon AV amplifier (AVR-1906) misses its subwoofer! (protection circuit)

[Alonso1973]

Hello guys. I need your help and Ill make a long story short.

I run a AV + HiFi set up in parallel; meaning that 2 amps share 1 set of front floor-standing speakers (Mission V62): A Denon AVR-1906 powers the centre and rear surround channels and "feeds" a 2-channel amp (Pathos Classic MKI) via the PRE OUTS, this Pathos powers the front speakers for music listening when its connected to a CD player and also amplifies the front left and right channes when watching movies or TV. The sub (REL T3) is connected to the Denon also via a PRE OUT.

Why do I do this? Because the Pathos and the Cyrus CD6s connected to it are much better at the music department than the Denon and the Panasonic DVD player attached to it and I dont want to have 2 sets of speakers for each system.

Now, yesterday I disconnected the sub and guess what happened. Remember the scene in Ratatouille that the old lady shoots he shotgun inside the house? Well, doesnt matter, the thing is that when playing fairly loud (0-5dB) during the first shot, the amp goes into protection mode (basically shuts the speakers off while the power button light blinks orange)

Besides disconnecting the sub, Ive done nothing different. The instruction manual mentions that this mode will be activated either because of a short circuit, overheating or when the unit is run at higher volumes for long periods with speakers of lower impedance than the one specified (less than 4 ohms)

The question is WHY? Why would an amp go into self-protection mode when there are no short circuits o over heating? Do you see some logic? Would it be that if the amp doesnt 'see' main speakers nor a sub in the pre-out section, higher transient signals makes it shut off?

Your advice, input, comments, etc will be more than appreciated!!!!