That is usually power sitting in a transformer and getting the power discharged as soon as HP's are plugged in.
Try this and this is what I do myself. When turning off your system turn off your tube amp but leave the HP's plugged in and music feed into the amp. You will hear music playing even tho the amp is turned off. The music will get quieter and quieter. Do this for 5 minutes then pack up the rest off the gear. What you have just done is discharged all the power from the transformers and tubes. When you start up the next time you shouldn't have a problem.
Don't adjust the volume when turning off. Let it carry on playing.
When starting up have HP's plugged in from the start. Tube amps always want to have a output. Whether it is speakers or HP's.
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No! An OTL (Output TransformerLess) amp is very different than an Output Transformer Coupled amp such as the Woo WA22 . Yes, transformer coupled amps can be damaged if they are operated without a load. But it doesn't hurt an OTL at all.
In an OTL amp, the headphones are directly connected to the plates of the 6AS7, with nothing in between. Leaving the headphones plugged in from the start, the start-up surge can easily damage headphones. This is not just theoretical. It has happened. And repairing or replacing headphones can be expensive.
So again, for those who own OTL amps, never turn the amp on or off with the headphones plugged in. And again, after turning the amp on, wait several minutes, until the output tubes have reached their normal operating temperatures and stabilized, before plugging in the headphones.