That is my understanding as well, the preouts on many AVR's is truly low voltage, but with that much voltage out of the pre/pro maybe it is the problem? The fact the Denon would need to be at over half volume for the same output out of the amp is just due to the fact the input voltage is lower and I highly doubt you were missing anything, the signal quality was most likely not any less just a lower voltage. Maybe try the Denon back in the equation if you can.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70