I've been a Sennheiser fan for some time now and have owned a pair of HD800's for some time. I was using AKG 702s for secondary systems but recently bought new into HD650.
Unbalanced they sounded slightly better than the AKGs but not dramatically so. That is until I made up a balanced cable and connected it to the main rig - the AQ is astonishing. Not quite as good as the HD800's but with heaps of air, clarity etc. Most impressive. Here is the system they were just auditioned in:
Balanced digital input from a Meridian Sooloos server -> M2Tech Vaughan DAC -> Balanced HeadRoom Blockhead amplifier -> new balanced cabling -> Sennheiser HD650 headphones (usually HD800 headphones)
The headphones and constructed cable will be used in this secondary system:
Oppo CD/DVD (modded older model) or Sony Blu Ray -> digital out -> (used & on the way - not here yet) Woo Audio WDS-1 -> (used & also not here yet) BUDA Headroom balanced amplifier -> HD650 headphones
The cable was purchased from my Meridian dealer in the UK - it is special CAT 7 LAN cabling used for Meridian speaker link connection. The Sennheiser plugs are Cardas, and required care about applying minimal heat. The cable has 4 copper twisted pairs so is ideal for balanced connection. The only downside is the cable is relatively inflexible but in the secondary system it will be used that is of no great concern and the way I made it there is flexibity either end. So I have a long headphone cable, constructed for well under $100 which is transmitting excellent AQ to the Sennheiser HD650s. If you can solder, try it out!!