Pre/pros are generally going to sound better because all of the real estate goes to doing processing only and no compromises have to be made in the way of allocating space to amplification. Then there is the fact that you use a separate amp, which also are typically going to be more powerful than what you'll find in a receiver for the same reason, all that real estate goes to doing amplification only and with all that room, they tend to be a bit more robust comparatively. The downside is, most pre/pros don't see the typical yearly replacement cycle like receivers, so you can get stuck without the latest and greatest. However you only need to replace the processor...
Outlaw Audio would be perfect for a side-by-side. Their 1070 ($899) avr is 65 X7. For separates they have the 970/7075 ($1299 for both). The amp is 7 X 75. The 970 pre/pro is almost identical to the 1070, minus the amp section of course. According to the specs, the 7075 amp is better than the 1070 avr in these areas. S/N Ratio: 119db to 100db, current draw: 1200 max to 855, weight: 41lbs for just the amp to 40lbs for the avr. Even I can see the separates have the advantage but I wonder if it's a $400 one.
I have a pre/pro setup, but I use a receiver as the pre-amp. I have a Rotel 1055 with a 1075 amp. Why did I choose this route? Well, when I bought this stuff a couple years ago, the 1066 pre-amp was the one closest in cost to the receiver. And the differences were negligible as far as sound quality. So for a couple hundred dollars less, I got the same great Rotel audio quality plus with the receiver I got a tuner (not included in the 1066) and I still got the amp section, which while not currently in use, could power 2 rear speakers should I ever get the room to go 7.1.
So yes, in general, a pre/amp combo will sound better than a receiver. But its not always the case.