It would be best to use a passive sub with outboard tube amp if you wanted to go this way. I don't think think the tubes would last very long inside an active sub..plus there would be heat to think of, not a good idea for active.
I've just been listening to the Seventh Veil loudspeaker system with Tom Evans' Soul (tube) Power amplifiers.
Initially the Soul amps were just used for the full-range speakers and an inexpensive solid-state power amps were used for the bass.
Changing to Soul amps for the subs as well as the mains made a dramatic difference top the bass, integration, tone and timing.
Of course the Soul amps have a far better bass end than many tube amplifiers and the solid-state amps used were relatively cheap and could, no doubt be easily surpassed.
Still, this listening session changed my ideas about amplifying subs (particularly with the Seventh Veil speakers). Ideally I would now go fo tube amps throughout, with the sub amps matching the main amps if possible.
You could make a SET with an 833A to power a subwoofer with enough power. Use the high voltage transformer output from the power supply of a discarded microwave oven and a full bridge rectifier consisting of four murcury vapour rectifiers. The output transformer might make steel cabinets and desks slide across the floor and you will want to have a well ventilated cabinet that will keep the 2500 volts you will be giving it where it will not be accidently touched. Some people at GR Research built one, but I think they did it to be funny. However it worked.
Personally, I almost became so disappointed with subwoofer amplifiers going bad in a year or less that I almost built such an 833A monstrosity. Fourunately, Audio Express came up with a sensible subwoofer kit that perfectly integrates with my 45 SET amplifiers.