Any tube subwoofers out there?

You don't see any tube subs because it is such a bad idea. Tubes can't control bass very well, tubes can't drive low impedences well and there is a loss of bass when you go through a mile of output transformer wire. And there would be lots of microphonics from the tubes shaking.

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.