Tube amps known to have a low damping factor?

I would think it much easier to make of list of tube amps with a dampening factor higher than 20.

(Limited) damping in tube amps is not due to the power output tubes, but to the magnetic hysteresis (lag) of the power output transformers. SS amps (*most of them) where the speaker (load) is connected directly to the power output transistors, can have damping factors of 1000 or more! (Which is why SS amps are prefered for woofers and subwoofers.

* One exception I know about (in SS amps) are those McIntosh SS amps which employ an 'autoformer' at their output. If you check the specs on those, you will see they show a rather modest damping factor, just like tube amps; but again, this is due to the transformers.

I think the information in the posts by by Duke and Nsgarch is worth keeping in mind.

There are speakers that are designed and voiced with specific amps or types of amps. That doesn't mean other amps can't be used, but the designer had a goal in mind when developing the speaker.

Here is an example. I own Music Reference EM-7 12v SET amps. They have 6 ohm taps. If I hook up the wires leading from the transformer in parallel I get 3 ohm output, series 12 ohm.

My speakers are made by Duke (Audiokinesis) and their impedance curve is between 8 - 12 ohm and very smooth. If I followed Stevecham's theory I'd need a solid state amp or a tube amp with a fair amount of negative feedback (like my Transcendent Sound T-16 with 1 ohm output) to satisfy the desired 10 to 1 ratio. Duke used the Atma-Sphere S-30 to voice his amp. It provides nowhere near the desired ratio due to it's highish output impedance (close to 7 ohms IIRC) and very low feedback (mine is configured with zero feedback). However, it's an outstanding amp for the speaker.

Getting back to the EM-7's. I've tried both configurations and prefer the 12 ohm output. As far as I can hear nothing is getting rolled off.

If all you want is a list you can start with most OTL amps, SET amps, or other Class A tube amps with zero or very low negative feedback. It will be a fairly long list.

Compared to ss amps most tube amps have lower damping factors. Wolcott tube amps appear to be capable of a rather high damping factor for a tube amplifier.
Some amps with a high damping factor have little or no global feedback.

In tube amps the loop negative feedback is a big factor in the resulting damping factor. For example an OTL can be built with a damping factor of 50:1 if you have enough feedback. IMO such a large amount of feedback does nothing to improve the resulting sound, and also it should be considered that no speaker made needs more than 20:1, and many of them like a lot less.

An example of the latter are open baffle designs- in order to get them to play bass the amp might have a damping factor of 0.5:1 or less and yes, that's not a typo. IOW, a *lot* depends on the speaker and the designer's intent!