Came here to also say don’t buy without listening first. To any product. I’ve heard a lot of both brands, have sold them both on sales floors, and think they sound almost opposite from each other, which makes me curious about your interest as those are two brands I would not think someone would include together in their final list of preferred brands to buy from.
To answer your question, yes there definitely are pieces to avoid. This happens with all companies to some degree. I would find forums for the two brands and learn from those communities. The irony, though, is it’s the less desirable pieces in which you will be able to find a score, and the best of a brand will likely always go for market value if not more.
Rotel will be the easier of these two to find a score on. The only way to score McIntosh, and it’s been this way for a long while, is to be the lucky person whose able to take something off someones hands for cheap as they simply don’t have the time or energy to post it up on an auction site. Those pretty lights go a long, long way. Lastly, if you haven’t considered other brands, I would open up your search. There are plenty of small brands that provide better sound than either, without the notoriety that would prevent a ‘score’ from being had.
Learning how to do diagnosis and repairs for common issues in amplifiers. Takes buying a few tools, multi meter, soldering iron, but you can save some serious cash on pieces labeled “as is” or “for parts” on FB marketplace, Craigslist, etc.