Hi!
There's no appreciable difference once you get in to the mid/high end offerings from any manufacturer. The only reason to use a separate amp is to drive low impedance speakers, or when you are using a receiver to drive surround sound and have full range towers up front that would benefit from their own power supply.
As usual it all depends. If you get a pre pro and drive it into amps with A/B output stage of equivalent power you likely will not notice a difference.
However all amps do not sound the same. So if you use power amps with a superior output topology and with significantly more power, then yes, what you suppose will likely be correct.
The other issue is that the pre/pro approach allows you to use active electronic crossovers. Properly implemented in the lower octaves, this approach results in a quantum increase in performance.
So as usual it all depends on implementation and the overarching design of the system which route is best for you.
I could not get a system close to the quality I have now if I used a receiver, not even remotely close.
Receivers just don't come with this much heft.