Interesting and definitely goes to show that perception of 'sound quality' is in the ear of the listener. I am certain that in a blind test one would be hard pressed to determine to which receiver brand was playing but in informal listening at big box stores I would not say under any under cirumstance that the HKs are 'bright'. Quite the opposite - I would say they were dull.
Funny, I was going to say something very similar (you beat me to it)
Anyhow, I find it very strange that ANYONE, can say that a AVR is bright, flat, etc..
Considering that if it is 'bright', then certainly there are ways to completely 'flatten' it out, by turning the treble (or any EQ in the AVR) down.
So, if I turned the treble all the way down on a HK, and all the way up on a Denon, then the person can still say that the HK is bright
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I know my Pioneer Elite has a 7 band built in EQ, and the previous ones I owned had either a 5 band, or some way to lower the higher frequencies.
So, at this point, what the OP may be hearing is 2 AVRs, that are not level matched (including tuning for the subjective ear, of the high and mid FR)