why don't tubed amps have headphone jacks?

Amps? I've never seen a headphone jack on an amp, tubed or solid state. In fact, you physically can't have one on an amplifier without adding a volume pot and an entirely seperate apmlification section (else you'd blow the cans). And if you did that, you'd have an integrated amplifier (with both pre and amp sections), not an amplifier. You want an integrated amplifier, fine, but that's a seperate beast from a straight amplifier.

As for headphone jacks on tubed integrated gear (or preamps), sure, seen it plenty. Generally, it's added cost and circuitry that many purists will prefer to exclude -- if you want a preamp, buy a preamp, if you want a headphone amp, get one of those. As price and quality go up, simplicity is typically a driving force as well. By doing fewer things, you can do those things better. But, folks definitely do it. I've had two tubed preamps (VTL and Rogue). The Rogue (99 Magnum) has a headphone jack. A pretty nice one, actually.

Does that answer your question at all?

Pawlowski, cool. That said, I had intended to mean nothing more than you wouldn't want to drive cans off of speaker taps (or the final amplification bits of an amplifier intended to make speakers go). Can't imagine that cans on 250 + watts would fair so well. Thus, however you get it done, I presume you'd need more than what you find in a vanilla speaker amplifier. But, clearly, I know just boatloads about "electic bits...."

Heh, look at me all learnin' and stuff.