( i also have standard apple, venturecraft original and vmoda lightning cables).
The OP wanted to know if going to 4 pin XLR would make the M8 sound even better. I would argue no, or at least barely. The M8 is unique, or maybe rare, in the since that the 1/4" jack gets the same amount of power as the "balanced" output. Most balanced amps get major power increases through the balanced outputs.
I agree that the HD650 can be made to sound better with a cable upgrade. Its just this Amp/DAC has nothing to do with said improvement. Also an ergonomic improvement, if the long stock cable makes things difficult to move around.