I need a 20 or 25 foot headphone cable

I use a 20ft. Stefan AudioArt Equinox cable with my HD 600's. Prior to that I was using a 10 ft Cardas cable with a Grado extension cable. I would try to avoid an extension cable if you can. The weak link is the connectors in the cable. You're much better off with a long cable (no additional connectors). But once you plug them in I would leave them. Plugging unplugging, plugging unplugging, etc. will take its toll on the 2 prong pin plug to the headphones. BTW, the Equinox cable was a big improvement over Cardas. It's pretty expensive, and a long wait to boot, but worth it IMO.

Drew at Moon Audio will either make you a Senn cable that is long enough for your purposes, or sell you a headphone extension cable. He makes 'em in three varieties I think and they are all of excellent quality. Not a long wait either. http://www.moon-audio.com/
Personally I would go for a long headphone cable, not an extension, as I tend to accidentally pull the plug out of the extension if I move around or lean over to pick up the remaining sections of newspaper I am reading etc.

The Moon cable is very good. Never hear the equinox.

I'm running a Revelation Audio Labs cable with my AKG 701 and have found it to be, by fare the best I've heard.

Which for me means it's like no cable at all. It has no sound of it's own. It just gets out of the way and lets what's actually there come through.

Brad over at REL will custom make whatever cable set-up you desire. ie. Total re-cable or just an extension in whatever length you want.