The connections on DL33 would appear to mirror the feeds to the 621 Producer's panel, with Stereo Cue, Aux 1(mono), Aux 2(mono), and Quad Bus LF/RF.. Patching in a more elaborate headphone system is inevitable. Around here, we have the Mytek Private Q system, with it's 8 mono and 2 stereo inputs, all 12 channels coming up to separate user option patch points. As such, the engineers here are free to derive headphone mixes from the cues, tape returns or even the small fader sends to the routing matrix, with the small fader in Input mode. Plenty of ways to skin this cat.