I know many of you don't believe that cable swapping is worth it for sound quality unless switching to balanced. Considering that HD6XX is a rebranding of 650 and costs 200$ less, the cable would be an obvious compromise. So I ordered the HD650's official replacement cable. I was not going to expect any massive changes to sound, but was still curious to try out. I tested with FiiO K5 Pro and 320kbs mp3 files (not a high end setup, but certainly not bad).
The 650's cable determinates to 6.5mm jack instead of 6XX's 3.5mm without the adapter. I don't know if using the adapter could affect the quality of signal.
As for the sound difference between 6XX and 650 cables: As expected, the difference was not dramatic, but noticeable by trained ears. The bass response sounded tighter and punchier with 650 cable. This made music in general sound slightly cleaner and more dynamic.
But realistically, the cable switch is not at all necessary with 6XX. But if you want a tigher bass to improve clarity, the 650 cable could give you just a tiny improvement.
Sorry mate, but you're not hearing a difference, you're imagining you're hearing a difference, the only difference you could hear is if the cable get's too long, you can calculate it with this formula:
fC = 1/(2 ⋅ π ⋅ R ⋅ C ⋅ 10^(-12) ⋅ d)
fC is the max frequency of your signal
R is output inpedance in Ohm
C is Capcacitance of the cable in pF per meter
d is cable lengh in meters
I 100% guarantee you that you're not going to get anywhere close to 20khz when you put the numbers in for your cable
For anyone too lazy to calculate it themselves, even if we put pretty much worse case numbers in, like 200pF per meter, 5 ohms output impedance, 10 meter cable lengh, we reach a maximum possible requency of 15.915.507 hz
So yeah, not even close to the cable affecting sound in any way