I'm sure the reply line is a traffic jam when it comes to the issue of cables.
I'm a tested and veteran cable skeptic myself. Work long enough in a studio and doing critical live field recording, you certainly want reliable and solidly built cables.
The only reason I have replaced the HD600 and the HD 650 stock cables is because
the connectors failed often enough during session work. It became a nuisance
to break the session while I yanked the headset off and squeezed the pins back into place so they resumed contact.
The personal caveat has always been with Sennheiser: How can you sell such an expensive headphone with such a flawed cable connection? What gives?
As for enhancing or improving the sound of your Senns with a $250 cable?....Pulheeeze.
Electricity, conductivity, and signal path purity are NOT going to change with your 300 plus- clam upgrade. We build our own cables from 6ft. mogamis with neutrik connectors and the cardas end pins for a fraction of the cost and the "sound" is indistinguishable from the boutique miracle cable selling for the cost of your cans.
It is possible that some cans will improve with certain types of cable combinations depending on the impedance of your cable, but these delicate tolerances are still subjective.
That said, well-crafted cables have some measurable advantages and a skilled cable builder can make you a palpably superior physical product than that whimpy spaghetti noodle of a cable that comes with the HD600:
primarily, the quality of the copper strand assembly and its shielding with components that will minimize interference whether, magnetic or IRF. Secondarily, the quality of the casing, joint sleeves and soldering skill can make a cable sturdier and better resistant to constant movement and fraying or tearing. One wants a cable that can take the real life, mano a mano handling, and storing every trip. No fraying, twisting, signal drop, or cable fold failure.
Know what I mean?
Save a few bills, and buy some killer recordings on CD. Brew a cup of Colombian and put your legs up, turn on the DAC and pump the gain where you will start to cry with emotion as the music washes over you like a tsunami and you are wiggling your hands and toes to the groove of the music. That's the MUSIC, not the cable.
Happy morning to ya.