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 appreciate that people are willing to share their expertise... but I'm just hoping that someone will be generous enough to share the fruits of their labors so I don't have to do the work myself. A list for the rough parameters indicated is all I'm looking for.
Plenty has been written on the subject of impedance matching and the relationship between damping, feedback, distortion, etc... no need to reinvent the wheel. Thanks.