You're asking this question the wrong way. It should be "Is an OTL (output transform-less) tube amp really worth it over solid state amps?" And the answer would be, "An OTL tube amp is not routinely or predictably better. But when a given high impedance headphone responds well to a high impedance amp, the results are things you can't get with SS."
Just last night I listened to a heavily modded HD650, the Jupiter Audio Research "JAR650," on 2 rather different amps:
- Woo WA3 (OTL tube design)
- And the Liquid Carbon v2
It sounds fantastic on each one, but doesn't sound the same on each. On the OTL there's a small but very good sounding increase in level/calm/balanced reproduction of frequencies, top to bottom. A little hard to describe, the but the HD650 sounds ruler-flat, controlled, and musically accurate on the Woo OTL.
The LC v2 is a SS amp clearly "voiced" to be somewhat warm & "tube-like" (and it's all that). On the LC v2, the HD650 doesn't sound conspicuously warmer. The bass might be a little bit deeper/more impactful, and the entire frequency range is a little less well controlled & damped...sound is a touch more exciting, for lack of a better word.
IMO the main differences between the sound on these 2 amps is that on the OTL, the HD650 is getting a high impedance source, while on the SS amp, it is not.
I've heard roughly similar results with 2 other high-impedance headphones on SS vs OTL (ZMF Eikon & Aeolus), where the OTL sound is somehow more controlled, damped, level.
The punchline of all this is anyone w/a high impedance headphone like the HD650 owes it to themselves to hear it on an OTL like the WA3 or Darkvoice. But the HD650 will also sound terrific on any competent SS amp (I've tried this HP on 5 SS amps--all sound great).