What I meant by "externally" was to place the resistor in a headphone extension cable. This could be done separate from the Taboo MK III.
By purchasing an XLR jack that is male, and a second XLR jack that is female it would be possible to wire a resistor between the two jacks and experiment with different values. Once you find the value you want, you can order that value from mouser using the part number I supplied for best sound quality.
As for internally modifying your units, I don't have a problem working with small repair shops via email to guide them in what to do, but I am reluctant to post what to do on this public forum. There are dangerous voltages inside not to mention working on it yourself could void the warranty.
It should also be noted that this is not a fix, it is a mod. You can't fix what's not broken, and for those hearing hum on non LCD2 headphones, this mod would likely help a great deal.