Does anybody, by chance, know an TRRS jack that would fit into T50s instead of the standard jack socket? I've got an idea of minimising common ground resistance, which produces unwanted crosstalk, by removing any non-soldered contacts from its domain, using 4-conductor cable and TRRS jacks both on amp and headphone sides. Cable and amp-side is not a problem, but have suspicions about phone-side jack.
The T50RP's use a standard Switchcraft TRS enclosed jack with a simple plastic frame to keep it in place. There is no drop in replacement from Switchcraft or any other manufacturer that will give you TRRS. There are a couple panel mount TRRS jacks available, but they would require heavier modification and may not fit since they look longer than the Switchcraft.
Someone else asked me about using a TRRS jack for the same purposes, potentially adding it to the bottom of the cup (going up into the normal jack space). I still think that an mini XLR is a better idea. More robust, locks, and would probably take up less volume in the cup (which would be ideal for balancing the sound).
Here a TRRS jack I found: http://www.showmecables.com/product/35mm-trrs-jack-panel-mount-connector-metal.aspx