Subwoofer tube amplifier?

Don't see any out there.

Does everyone just SS for their subs?
Is there no point in it?
Would any 'ol amp suffice as a separate subwoofer amp?

There might be a need here, maybe.

Solid state class D seems to rule in sub amps that usually are rated at multiple 100’s of watts.

I think two main advantages:
SS Watts are cheap and efficient.
SS amps generally have a much better damping factor which is desirable at lower audio frequencies to control woofer cone extension.

Combining above, a SS subwoofer amp is going to work better and be waaaay cheaper than a valve based amplifier.  

Madisound sells a bunch of plate subwoofer amps, Hypex and Rythmik both sell DIY friendly products.

And to get the desired low frequency responses would require huge amounts of iron in the output transformers......