Ok. I am now a believer in burn-in. Out of the box, the bass drum / kick drum representation of the HD555's sounded like a lollypop hitting an empty cigar box and the HF was pretty grainy. Following the recommendations on this thread the HD555's were connected to my Zune and let run on shuffle for 72+ hours. Now the bass is much improved. It is not as forward as the bass on the Denon D1000's but those are closed phones. While I doubt if it is reference quality, I like it. Here is a link that you can use for comparison of different phones.
http://www.wooferbasstest.com/techno-bass-test/subwoofer-test.html
Overall, I am very pleased with the HD555's. They really shine on vocals and acoustic music (Allison Krause, Diana Krall, Norah Jones, etc) and don't do too bad on modern jazz (Spiro Gyra, Kenny G.). I was really impressed with the vocals and instrument placement on the Eagles, Long Road out of Eden. (It was the kick drum on this song that sounded so bad in the beginning.)
I thought I would listen to them for awhile before doing the 595 mod. so I could get a good feeling for the change / improvement in bass. They sound good enough now to enjoy the listening experience. Now I have three different sound signatures - Er4Ps. Dennon D1000 and the HD555's each with their own flavor.
Listening environment: Netbook w/ VBR WMA -> Corda Move Dac -> HD555's