Hi, Sov.
The RAL re-cable on the Proline 750, IMHO, makes much more than a subtle difference.** Also, I don't think of headphone re-cables--or any audio cables for that matter--as "tweaks"; in my mind they're analogous to good speaker cable feeding loudspeakers. What you get from the source via the speakers is critically connected--literally--to the cable inbetween.
In a word, I consider cables as audio "components" in their own right.
Regarding two cans using the same driver and sounding very different from each other: simply interchanging the earpads between the Proline 750 and 2500 makes for profound differences in sound (which happen to be mutually deleterious, BTW).
However, if two cans, e.g. the Ed.9 and the Proline 750, use the exact same driver, then I have a very hard time rationalizing paying an extra $1000 for non-driver-related materials and design. Admittedly, although I have not personally auditioned the Ed.9, they may sound leagues better than the Proline(s) but I still can't justify the stilted price for what is left--after the drivers--for Ultrasone to do. After all, it could be something as simple, relatively inexpensive and seemingly minor as the earpads that brings about such a sweeping improvement in the sound (i.e. of the Ed.9 v. the PL750). But then again, I don't think it warrants a $1000 price increase.
I'll be the first to admit that Ultrasone makes very special headphones; these Bavarian folks are obviously good at implementing their design. But they're also pretty good--and quite unabashed--at advertizing, too. : )
More than ever now, I'd like to a/b the RAL/Proline 750 v. Ed.9!!! : ) : ) : ) Until I stand corrected, I don't think I'm missing THAT much by foregoing the Ed.9 for the RAL/Proline 750. . . . at least for the time being . . . That is, if I can spend $200 for a re-cable that gets me, say, 90% of the way to Ed.9, then I'll be happy (and $800 richer).
(And yes, I admit, too, that the last 10% is definitely worth the listen. I think the "high" high end is all about achieving those last few, incremental, single-digit percentage improvements. The plot of % improvement vs. dollars spent becomes logarithmic at this stage of the game.)
**IME, the RAL cable significantly enlarges the headstage; renders more air and detail; produces deeper, punchier and more articulate bass; provides cleaner, better-differentiated highs; overall, a smoother, more organic and engaging listen, while still preserving that "directness" and visceral connectivity of the Proline 750 that got me hooked on Ultrasone in the first place. : )
PAB