As PENG pointed out, the Pre-pro by itself will have the better SNR because it does not contain the amp, which is usually the weakest link.
For an AVR, it is always connected to the amp, so the SNR will be lower. But if we compare the pre-pro + amp as a whole system vs. the AVR, we will see that the AVR may have better overall numbers.
Sony STR-DN1000 $400 AVR = Pre-Pro + Amp Combined:
THD+N was less than 0.010 percent at 1 kilohertz when driving 2.83 volts into an 8-ohm load. Crosstalk at 1 kHz driving 2.83 volts into an 8-ohm load was –92.18 decibels left to right and –86.60 dB right to left. The signal-to-noise ratio with 2.83 volts driving an 8-ohm load from 10 Hz to 24 kHz with “A” weighting was –107.74 dBrA.
Sunfire TGP-5 $3,500 Pre/pro:
THD+N is less than 0.017 percent; Crosstalk was –88.69 dB left to right and –88.49 dB right to left. The signal-to-noise ratio was –123.11 dBrA
Sunfire TGA-5400 $3,900 Amp:
THD+N is less than 0.020 percent; Crosstalk was –76.83 dB left to right and –75.98 dB right to left; The signal-to-noise ratio was –100.49 dBrA;
Sunfire $3,500 pre-pro + $3,900 amp = $7,400 combined:
THD+N is less than 0.020 percent; Crosstalk was –76.83 dB left to right and –75.98 dB right to left; The signal-to-noise ratio was –100.49 dBrA.
Your system is only as strong as it's weakest link. The weakest link is usually the amplifier. So even if the pre-pro has the SNR of -123dB, the overall system SNR is only -100dB in the pre-pro + amp combo.
You can see that this $400 Sony AVR has better measured specs overall than the higher-end Sunfire combo.
But, of course, not all Sony AVRs are that good. I've seen some that are bad too.
This was just to give example of why not all high-end pre-pros will measure better than all AVRs.
