I am in the same predicament...
I have used the Sommer cable (Peacock double balanced) on my Sennheiser HD650... it was difficult to get it nicely in the connectors an the jack, and quite heavy... i will not recomment it...
Alternatively i have used a Mogami Neglex 2534 (starquad balanced), easy in the connectors, easy to get it in the jack, the stripped two twisted pair part is very flexible, the cable itself is heavy and not very flexible... so although i am not exactly happy it does the job at the moment (not on the HD650's which i have sold now, but on my modified FostexT50rp.)
Sound wise i liked both they are on the warm side, which i like very much.
I have used also a ready piece of silver plated copper HiFiMan new type headphone cable, easy to manipulate lightweight (looking very snazzy), but little light on the lows and some kind of treble imbalance...
I am also looking into some kind of pure copper (no silver plate, i am not fond of the treble issues) starquad is the easiest to use, but i am not afraid to braid/wind single cables...
I am going to use these cables on both my HD800's and my Fostex's
Someone suggested the Mogami micro cables... has anyone tried them?
Any help will be appreciated