SPL meters are not accurate enough to prevent level differences causing audible tonality differences (not heard as a level difference). The only way to level match accurately enough is with a test tone and voltmeter at the speaker terminals. And then you need to be targeting 0.1dB - though 0.2dB is probably good enough. This is tricky if your volume control has only 0.5dB steps. You need to get fine grained analogue volume control in there somehow.Everything was level matched with spl meter,
And if you are not comparing blind - you can't stop your brain making stuff up based on what you know you are listenting to - even with familiar material. We can't turn off our perceptive biases.
And your blind testing needs to have sufficient number of trials between each pair of devices for statistical relevance (eliminate false positives from guessing).
It is not that easy to carry out a sufficiently well controlled blind test to give a convincing result.
I think it is also fair to say that all AVR's are doing some DSP - even if EQ is off, and especially if using surround sound - there are all sorts of settings that impact the sound - that might not be the same from one AVR to another - the only way to be certain of avoiding this is with analogue input, and whatever the equivalent is of "pure direct" which by passes any A-D and D-A and the processing in between.
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