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.
I think the 9 volts is more like a headroom thing. You might be playing along at .5 to 1 volt and then the music demands more where you can swing to 7 or 8 volts. By having that ability you get everything in the music. I think that the low voltage receivers may be losing some of the detail by not being able to produce the needed voltage. Here is a quote from a review on my preamp pertaining to the 9 volts:
Home Theater HiFi "The fact that it will output (a.k.a., "swing") 9 volts is important, because you might be using 0.5 volt on average, but along come those darn transients, and "boom", you need the ability to raise that output voltage significantly for a very short time. The USP-1 can do.
