Any tube subwoofers out there?

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

Yes But there DIY,my pair are horns over 8ft tall have dual 18in ea and are 103db 1 watt but can handle 500 con.No sub on the market is even close to there performance.

With plate amps like the Adire ADA series, why go passive with tube amps? You have 2nd or 4th order low pass options (for sealed or ported monitors) and 2 bands of parametric EQ for each channel (if a stereo pair - which I recommend). Maximum flexibility to integrate with monitors, allowing them to roll off naturally on the low end. I use them and I'm most satisfied. My bass units are not subwoofers either - Focal 10W6452 10 inch woofers that contribute measurable output up to 200 Hz using 2nd order low pass with sealed monitors (Rat Shack analog meter). The monitor midrange/treble is startlingly clean using Atma-Sphere M60s, when freed from meaningful bass output (paired PHL 1240s in TMM). I gladly accept a measured 5dB dip between 100-125Hz as the price paid for otherwise flat in-room from 40Hz - 300Hz, as I don't notice it on music. I don't obscess over sub-35Hz for music anymore because there isn't much content that low and I'd rather not have to tame any more room modes than I already do. Going this route isn't for everyone, but eliminating a high pass crossover unit to the midbass drivers and sealing the monitor cabinets has audible benefits.