I have 3 AV receivers in my house that I am auditioning right now, a 2006 Yamaha RX-V2500 that originally listed for $1100, a 2004 Pioneer Elite VSX-55txi that originally listed for $1800, and a 2008 Sony STR-DA5400es that originally listed for $2000. The $1100 06 Yamaha RX-V2500 sounds better than both of the more expensive AV receivers, the 08 $2000 Sony 5400es sounds the worst of the 3, even though the Sony got rave reviews from every review online I have read about it. Sound & Vision even said the Sony 5400es was the best sounding receiver they tested that year, if not the best AV receiver they ever reviewed, I don't get it.
First off, I don't think there is a credible answer to your query. Too many variables and unknowns.
But, its the kind of question one would naturally ask if you found yourself with 3 AV receivers and you stacked them up and listened to them and 1 sounded better than the other two. Why indeed? I would want to know.
First off is age. A 17 year old receiver vs a 15 year old verses a 13 year old. Where have they been all these years?
There's nothing inherently wrong with age. Well kept and cared for there are units running far longer than those.
But age and condition is a factor.
Second off, did you have any Audessey or room correction SW running? Did you do any tweaking to any of them?
How did you level test their outputs? A very slight volume difference will make them sound quite different and its tough to level match products by ear. You'd have to use a meter or measure of some type.
These are just the first things that come to mind. I'm sure the amplifier experts here would have a far better set of questions to ask and opinions to offer. I would want to know just like you want to know.
But, I think a lot more info would have to get developed to really develop a working theory