I've done this in the past to check for sound characteristics at low volumes. This was to see if what I was hearing was caused by the amp, or speakers. I would not want all of these amplification stages altering the sound in my headphones though, when a headphone amp doesn't need or use them. But for safety reasons, I would use a 1/4 to 1/2 amp fuse in each channel.
Others with more electronic knowledge might give a more appropriate value. This would be in case something happens to go wrong for example, an output transistor shorting and passing 50 volts or more to the headphones. Besides the loud volume, the phones can catch on fire (if not protected), with all of that power there.
If people reading this are considering doing this on a tube power amp, make sure you keep a reasonable load on the speaker outputs on amp at all times. I would guess a 8 ohm non- inductive capable of handling the output power from the amp.
Others with more electronic knowledge might give a more appropriate value. This would be in case something happens to go wrong for example, an output transistor shorting and passing 50 volts or more to the headphones. Besides the loud volume, the phones can catch on fire (if not protected), with all of that power there.
If people reading this are considering doing this on a tube power amp, make sure you keep a reasonable load on the speaker outputs on amp at all times. I would guess a 8 ohm non- inductive capable of handling the output power from the amp.