Was that sales person telling you the truth? Or was he manipulating you when "there was a moment of silence, and then he said, without any enthusiasm, it (the Denon 4700 AVR) should be Okey"? You were there, and I was not.
I finally found the manufacturer's specs for the Focal Aria 926. It is no longer available, and I found them
published in a review. Usually that magazine/web site doesn't measure anything themselves. They simply repeat what Focal told them.
Sensitivity: 91.5 dB (estimated at 2.83V at a distance of 1m)
Nominal impedance: 8 Ohms
Minimum impedance: 2.9 Ohms
Recommended amplifier power: 40 – 250W
Crossover frequencies: 290Hz / 2400Hz
We have already discussed this speaker's low impedance of under 3 Ohms in the frequency range of 80 to about 500 Hz. With a woofer-to-mid range crossover so low at 290 Hz, it will make large demands on any amp. I remember
@TLS Guy commenting how he thought that this feature made that speaker a poor design.
The speaker's high sensitivity of 91.5 dB is very high. Sensitivities of 86 to 89 dB are more commonly seen. I have to wonder if Focal measured that value in the range of 80-500 Hz where the impedance is so low. There are no industry standards for measuring or reporting a speaker's sensitivity. Let the buyer beware.
I'm going over this just to say, it is possible that your Focals did over stress the Denon AVR. Your Denon definitely did fail, but we'll never know for certain if the Focals caused them to fail. It also suggests, but does not prove, that the Denon's amp section was less robust than the Arcam integrated amp, even though they have similar power ratings.
Finally, Focal recommends the Aria 926 be driven by an amplifier that delivers power in the range of 40 – 250W.
@PENG had previously noted that. It becomes more & more clear to me that your Focals are more power hungry than your Denon can deliver. Look for an amp that triples your present power of 120-125 W.
Power is not only about how loud you want to drive your speakers. It's about how clean your speakers can sound with higher power at any loudness level – without causing the amp to become stressed or go into clipping. It also depends on music preferences, and all those room variables we discussed earlier.
In a perfect world I'd recommend multiplying your amp's power by 10-fold, from 120 W to 1,200 W. In math speak, boost the power by a full log. But in reality, that's too expensive or not even possible. So, we settle for boosting the power by a half-log, the square root of 10-fold, or 3.16-fold. Triple is good enough.