Assuming your Purifi is actually the 1ET400A per Urgo or Hypex's NC400, there could be several reasons why you heard no difference, and/or no change in level settings by Audyssey:
For the unchanged level settings part:
- You forgot to send the new Audyssey Mult EQ results to the AVR after the re-run on the MultEQ Editor App from your device (smart phone or tablet)
- Your Amps come with some sort of extra gain stage, bringing close to the Denon's approximately 29 dB gain (28-29)
- When you did the re-run, the mic position might have been different than when you ran it without the external amp.
For the heard no difference (assume that's what you meant..) part:
- As expected, it is very hard for humans in rooms with the average kind of noise floor to hear distortions+noise below -80 dB when listening to music and movies.
Regardless, unless you did the comparison under the exact same conditions, using the same media contents, and switch between the two being compared in very short time if not instantly (such as a few seconds), and did it without knowing which one you are listening to, any difference you heard would not be reliable anyway.
Between the X8500H's and the Purifi or Hypex amps, all else being equal, it would be very tough to hear any difference if "power output" is not the issue in the first place. Anyone who claimed hearing the difference would disagree, and no one can tell them what they heard wasn't real but.....