why don't tubed amps have headphone jacks?

My Audio Space AS-3i integrated has a headphone jack. A nice one, too, not just an afterthought, and that's one thing that tipped the the scales in its favour.

Mezmo, you don't need a separate (opamp) amplification circuit. The Cary 300SEI (integrated) has their headphone jack wired directly to the output transformers. That's why this particular amp is so coveted by headphone devotees.

The Cary SLI-80 integrated has a very nice headphone amp.

I purchased a Cary Audio SLI 80 F1 for a second system. The main reason was it has a really nice headphone section and jack.

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...."