As I mentioned in my post, I’m talking about an XA25 speaker amp fronted by an XP-12 preamp. I use a custom adapter that goes from spades to XLR female. The Susvara is particularly inefficient for aheadphone (83db/mw, 60ohm) and is particularly power hungry. Using a speaker amp provides performance that is so clearly superior (IMO) to what I have experienced with conventional headphone amps. To be clear, I am not advocating this for typical conventionally or very sensitive HPs.
As @Polygonhell mentioned, power is reduced at higher impedances. A solid state amp at 8 ohms will probably only have about 1/8 of it’s power at 64 ohms. So a 25W amp at 8 ohms will only have about 3W at 64 ohms. So it’s not as extreme as you might have thought.
Although I wouldn’t go so far as to say that headphone amps “often” are more powerful. My XA25 is actually measured at 80W into 8 ohms per Stereophile. That would work out to 10W at 64 ohms. I can’t think offhand of a headphone amplifier that can produce that much power into 64 ohms.
The Bakoon 13R is effectively a speaker amp that can also drive it’s full output out of it’s headphone jack.
I also agree that for a OTC tube amp, you can’t use a high impedance headphone without something like a parallel resistor. Second picture below is an example of what I did with my Primaluna Prologue tube amp to provide a parallel resistance for the amp close to the expected 8 ohms.
