Well, I did send the HD600 back. A fantastic headphone, and I would agree with many that it is a great bang-for-the-buck.
But, for my tastes, I found that I missed the bass in the HD600 more than I missed the mids and openness in the HP150. When listening to the Senns I kept catching myself thinking, wow, I bet that bass would sound fantastic on the HP150's! And off they would come.
Now, if I could just quit coming back to head-fi, I would be satisfied with the HP150.