Here is a version of the SSL DL59 pinout. Bear in mind that they changed a lot over the years though.
If you are listening to stereo cans over the headphones via the 621 Producers Panel, the signal you hear may not be the same as any musicians hear as they will probably be fed their mixes via a power amp, headphone amp or other mixer and would have a different sound.
Hope this helps for now from Jim Lassen