Any tube subwoofers out there?

Why not buy a good passive sub and power it with a tube monoblock or single channel of a stereo tube amp?

You can use bass guitar amp instead.

I appreciate the response of Rwwear because that appears to be a reasonable answer as to why not. Some of the answers as to how it might be achieved intrique me and are worth considereing. .
My listening is much more geared towards the higher frequencies than bass in general, strings, flutes, piano, yes it can do bass as well as can cellos or basses. More bass is just not an adictive route for me clearer highs are. More depth is also appreciated. More responses would be as well.

If you use a powered sub and cross it over very low, say 60hz or below it should not interfere with the upper frequencies except to give the system a feeling of warmth and more body. I personally feel that you should never use the xover for the main speakers to keep the mids and highs as pure as possible. Therefore the sub can use the most efficient means of amplification as possible. Which is now it seems class D.

I agree with both of Rwwear"s posts, especially the part about using a crossover on the main speakers, they should be used full range for the best sound as Rwwear says. Usually tubes just don't do it for typical subs but of course there are exceptions. If you have a large room and could build a large horn subwoofer using a highly efficient driver (98db or so) a tube amp would work fine. I've heard a few and they do it