That's design flow and it's annoying. The only way to get around it is to unplug speakers from amp. The output stage will still be loaded with headphones so your amp will be allright.
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According the owner’s manual for that unit, the speaker outputs are supposed to switch off when the headphone plug is inserted.
I would turn the unit off and gently mate the headphone plug with the jack. Make sure it is fully inserted because this is what breaks the circuit to the speaker outputs. Perhaps rotate it. Do this several times.
Then after enjoying a cigarette, turn the volume down, turn the unit on, and try it again. On my older Rotel pre, some gunk and dust prevented getting a good contact.