Have an old fashioned or a traditionally designed preamp or integrated that has a REC out output.....having to do with analogue or digital physical recording devices. While tapedecks have died out, my former LFD integrated had a tape loop, as well as my Marantz Pearl KI integrated. My circa 1989-1990 Classe DR-6 preamp has at least 2 of them, as well as my old circa 1989 Forte preamp has them. Loop the tape out into the headphone IN and then from the headphone OUT to the Tape IN of the in and you are in business.
Some people do have just 1 master source/front end connected to their headphone amplifier IN. Heck, I have seen some crazy setups, ultra reference, ultra expensive that used headphones and NO speakers at all. 1 source was it.