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?
