Bill,
I have a pair of HD-600s myself that I am using with a tubed Radii headphone amp.
There are a few things you can do for your headphone rig.
The first thing I would look at is your wall outlet. If you haven't changed it out, I would suggest the ACME 15 or 20 amp Cryo treated outlet. It takes about a month to fully break-in but it will give you better definition and more slam.
Nitro power cables for the amp and CD would also help. They are very open and have wonderful dynamics for the price. The Bybee Power Bar will help in lowering the noise floor and allow more of the subtlies of the music to come through with less glare.
Changing the stock cable on the HD-600s will also help. I do not make any cables for this but the Cardas headphone cable is supposed to be fantastic. I am a Cardas OEM and they were kind enough to let me order a cable for my own use. They are backordered at least 60 days, however.
Finally the biggest "tweak" would be to use a MENSA Plus modded DI/O between the CD player and the amp
I shipped Kris' (Obie) back to him on Monday. Maybe if you are real nice to him, he will let you borrow it when it is done with break-in.