For the HD650, I think balanced has a greater effect than the cable itself, therefore it is more worthwhile to switch to balanced configuration than just upgrading the cable imo.
Based on earlier comments, I tried to terminate the stock cable to balanced just to experiment. But I had the hardest time. It was like the wires were solder repellent. The interesting part was the part of the wires were died in red and green color i guess to indicate channel. Has anyone successfully balanced the stock cable? Could've been my iron or me just having a bad day I guess.
The cable used in HD-650 is a true litz wire. Every single strand is isolated. The isolation layer is indeed 'solder repellent'. You may succeed removing isolation with high temperature soldering, but it is not easy. Color indicates polarity (+ or - signal).