Tube amps known to have a low damping factor?

Which tubes amps are known to have a characteristically low damping factor, 20 or less? Which are known to have a characteristically high damping factor?
I would instead look at the output impedance ratio to the nominal impedance of the speakers the amp will drive. You want a ratio of at least 10 between nominal speaker impedance/amp output impedance. Anything less than this will usually cause a drop off in some portion of the frequency response, depending on the impedance curve of the speaker.
CAT JL-2 has a DF of 10. Why do you want a low DF? I thought I high DF was desireable...
Higher damping factors are not a free lunch; something has to be traded off to get there, and that something is the ear-friendly distortion behavior of a low (or zero) global negative feedback amp.

Most SET and OTL amps have a low damping factor, but they work fine on speakers designed with such amps in mind.

I suggest reading this paper before you select an amplifier; it will be helpful regardless of what type of amp you go with: