I just let the Signal Cable Silver Reference Sennheiser cable break in for about 100 hours.
Surprisingly, I actually prefer it on my HD600's, as opposed to my 650's.
I am using a Stefan Audio Art 650 LE Equinox on my 650's, and the Equinox sounds slightly more natural on my 650's.
The Signal Cable does sound really good on the 650's, but is very slightly thin sounding compared to the Equinox.
The bass and treble extension is quite similar.
The situation is reversed on the 600's.
The 600's sound better with the Signal Cable compared to the Equinox.
With the 650 LE Equinox, the 600's sound sort of congested or confused, for lack of better words.
The Signal cable opens up the 600's, and improves most every sonic aspect, and has less harshness to the upper mids, and more body to instruments.
With the Equinox on the 650, and the Signal Cable on the 600, the headphones sound similar, but the 650 is still slightly darker, and slightly bassier, and the 600 has slightly better midrange and treble clarity.
I was quite surprised by all of these results.
Both of these aftermarket cables sound better than the stock cables, but I am not sure if they are worth the relatively high prices.