I actually use a Stax SRD-7 that connects to the speaker binding posts of my amp. The SRD-7 allows me to switch between speakers and headphones. I have another device that allows me to do the same with dynamic headphones, but no switch so I have to swap the speaker cables between amp and the "box."
Would using the tape output of your preamp to a dedicated headphone amp would be an option?
Would using the tape output of your preamp to a dedicated headphone amp would be an option?