Thanks for all the replies! Keep them coming!
I have decided to keep the Supreme amp. I thought that would cure my "problem" but it did not. Many seem to think they have adequate bass. I used to think so. Then I got my Vandersteen speakers, and discovered the lowest octave of music. The Sony MDR7506's seem to match these speakers.
Now I have "broken in" the HD600's with 100 hours of music. This helped a bit.
I have also been using the Supreme and some good CD's to compare the MDR7506's, HD600's and HD570's. To my surprise, the HD570's fall between the HD600's and MDR7506's. My original thoughts were confirmed. MDR7506, good low tight bass, but mids and highs a bit restrained. HD600's... great mids and highs, no low end. Now HD570s, decent bass, good mids and highs. Of the three, I prefer the HD570's now, for overall performance.
I have also been studying the performance curves on Headroom website. The HD600's have a massive impedence jump about 100 hz. They also drop off in the low frequencies on the regular freq response curve. Going by my ears, and these graphs, I think my "problem" is the HD600's design, not a defective pair.
Love this tech talk. I used to be in electronics before going into computers. I have been into audio for 35 years.
I can hear a difference between cables in my system, but don't think that is the answer to my problem.
Many have mentioned the Beyer headphones. the DT250 series and the DT770 Pro. The DT770 has many recommendations, better freq and impedence curves than the HD600's. Given the nice Headroom return policy I think I may give them an audition.
I would also turn in the Supreme amp for an MG Head, but after trying the Supreme out as a portable over the Thanksgiving holidays, I love it.
So what do you think? Give the DT770's a try?