Most fully balanced amps I would say there is a substantial improvement going to the XLR jack, because you usually access the amp's full power potential. But not so with the M8. It is impedance balanced only. With my HD650s I'm hard pressed to hear any sonic improvements. I sometimes think I'm hearing a very slight soundstage improvement, and other times I'm doubting myself.
With that said, the stock HD650 cable is quite long, and not idea for desktop listening. So a shorter cable would help with ergonomics. And I could tell you about the benefits of silver plated copper or pure silver for the HD650, but I'm already hearing the groans for the other head-fiers...