Nothing has needed any replacement.
The ground connection on the 1/8" jack got finicky early on in the life of the cable (you have to bend it a little this way or that to get audio in both ears and with the channels separated), but in the 20 years after that problem appeared, it has not deteriorated one iota. You have to fiddle with it today with exactly the same pressure and nuance as in the year 2000.
I used the 1/4" adapter from time to time, like going directly into a synthesizer keyboard. Given 30 minutes to search around the house, I can probably find that thing.
I happen to also own a pair of 7506. Do note these are a closed set, and they are, by far, not comparable with HD600. Basically like night and day.
In the words of a coworker who also owns 7506, after trying my HD600: "Now the 7506 sound to me like an AM radio."
Still, I don't disagree they're decent as closed headphones at <$100, particularly if you only care about the mid range. But they're relatively colored, despite their popularity as "monitors", and much worse than the Japan-only mdr-cd900st classic.
I get more joy finding and using durable, decently performing products that provide excellent value. The art is learning, for any category of equipment, how much you need to spend to obtain good long-term value.