Protection circuit momentary activation. Not heat/speaker problem

Is it possible for you to lower the source level? if yes go ahead...

There is an automatic Dialogue normalisation function in Denon which is activated to reduce overly bright input signals. ( most movies have wide variation in sound levels and therefore this function is avaialble). (see page45 of your user manual).

When your system worked fine with the AVR 2808CI, then why is it giving problem with AVR 3808CI....???? You must ask Denon.

Protection circuits are activated when:
1). There is rise of internal temperature.
2).Core wires of the speakers are touching each other or a core wire is sticking out of the terminal and touching the set's rear panel or the speaker itself has a problem.
3). Speakers with an impedance other than specified are being used.
or
4). the set is damamged or has bad solder / dry solder connection .

If your AVR is new, please do not accept, the set could be faulty. The protection circuits should not be activated unless any of the above happen.

You must know, at times the protection circuits are not fast enough and it could burn some of the transistors on the power output circuit. With time, the solders on the circuit board could give way and give you problems later on.

You must sort out with Denon if the AVR is under warranty.

Best of Luck.