I think that you will be really happy with the cable upgrade over time. I really like Senns, but the stock 600/650 cable can be improved upon. Not dissing Sennheiser here at all, I respect designing to a price point and I continue to feel that they have the most refined and balanced house sound. And I think a used pair of HD600s or 650s is maybe the best bang for the buck deal out there right now. That they trimmed costs by developing an easily replaced cable and associated connector rather than using lesser drivers shows you just how savvy and dedicated Sennheiser is.
Now if they could just get their reps to use some listenable source material. I tried to A/B HD700s vs. HD800s at AXPONA and the music source was such incredibly SHITTY tinny bright headache inducing compressed MP3 pap that I had to tear the cans off my head and put them back on the stylish little headphone racks. Straight across from this was a perfectly fine demo of HD800s by, I think?, Music Lovers, with a nice little tube headphone amp and a rep happy to play whatever he had on his server.