I think the signal may degrade just a little, unless one of your external amp's input impedance are on the low side. My rationale is that, both Gene and Amir's alluded to the "fact" that the degradation was due to the power amp clipping when the pre-out voltage exceeds 1.2 V, and get progressively worse, to the point it would shutdown at 2 V in the lesser amps such as the Yamaha RX-A860, and RX-A1080.
So if they are right, and I doubt both EE would be wrong, then the culprit, at least the main/big one is power amp clipping, thereby sending back bad stuff the the preamp.
Having said that, if you connect two power amps via a Y-adapter, you will effectively lower the input impedance thereby increase the load to the preamps. If both external amps have high enough impedance such as 20 kΩ, then it is fine, but if they are anything less than 10 kΩ, it may degrade the signal a little, but again, at higher voltage only. In that case, the HDAMs may, or should help the situation. I don't know that for sure but I would imagine an extra buffer stage should help "buffering" the pre-outs a little better right? In fact, that may just be the only advantage of having HDAMs, that is, better buffering. In terms of distortions, it can only add, not subtract and that could be the reason why the Denon had better measured SINAD (definitely not audible to most humans) than the Marantz prepros measured so far.