The specs of the R900:
Tweeter | 0.75" Pinnacle Ring Radiator |
Mid-range/Mid-bass | - |
Woofers | 4” Turbine Cone |
Overall Frequency Response | 50 – 50kHz (when wall mounted) |
Frequency Response (-3dB limits) | 70 – 32kHz (when wall mounted) |
Recommended Amplifier Power | 45 - 100W |
Amplifier Output Compatibility | 8Ω / 6Ω / 4Ω |
Minimum Impedance (ohms) | 4.2Ω |
Sensitivity (2.83V/1m) | 85.5dB (when wall mounted) |
You are not wrong, the R900 can't possibly stress the Amp10.
I can see potential trouble in BTL mode because of the speaker's impedance characteristics (hard to know for sure without seeing the curves), but not in regular mode.
Absolutely wonky, but more information might help speculate the reason why your amps went into protection mode, such as you volume setting at the time, what did auto setup/Audyssey set the speaker levels (trim) to, what did it tripped on, that is, overcurrent, thermal, short circuit etc.? Also, which Atmos demo clip that tripped it, how do you know if only drive 6 channels and specifically the 900, what happened to the tower LCR speakers?