Generally, I agree with this concept (it's the same reason consumer car reviews are largely useless), but I don't think it's entirely applicable in this case for a couple of reasons:
1. It's not just new posters popping in to report problems. Regular and semi-regular posters are also reporting issues.
Maybe, but the forums still only represent a tiny, tiny percentage of owners who come here specifically because they have the problem, so it is not statistically possible to draw any meaningful conclusion from such a self-selecting sample.
I can't comment on the other makes because I almost never visit their threads. Pioneer and Yamaha are tiny in the world of AVRs by comparison with Denon and Onkyo, so one would expect fewer complaints, numerically speaking.
There is no evidence. Not really. Evidence of 'serious quality issues' would be knowing what percentage of total sales failed and nobody knows that. All that we have from the forums is anecdotal, and it is from a body of people who come to the forums simply because they have a problem. This is called 'self-selection' and it invalidates any conclusions statistically speaking. The bottom line is: we have no idea if Onkyo have a serious problem, or just the normal rate of failure for their industry as a percentage of total sales.