What you will/may notice when a tube amp is driving a speaker with low impedance dips is an uneven frequency response, e.g. the speakers may sound "bright".
I think that is exactly what worries me about a tube amp. Can it cover the low frequency impedance dips.
With solid state, one can form, at least, a preliminary sorting by learning the 8/4/2 ohm and watts specs; but it does not seem to be that easy with tubes. A few tube amp suppliers claim a 2 ohm load capability but most of the websites I have reviewed are silent on this topic.
Maybe I am worrying too much about it based on my Thiel CS5 experience.
Thanks for listening,
Dsper