Guess I'm a bit deaf or so, but I have the impression that SQ differences between amps tend to get boosted by the factor of 10 at most hifi forums. If some tube amp doesn't support impedances below 40 ohm, it might sound suboptimal, but such "oldschool" tube amps for ohm monsters have become quite seldom IMO.
Never ever crossed any amp (tube or SS) that made my Gradessandros or my 880'600s sound bad. Even in extreme bat ear mode, there weren't almost any differences. And if there were, it always turned out to be because of unproper volume leveling (a voltmeter attached with the load helps to avoid).