I do own both the Pro900 and the HD650 (also the ie8) I trading my Pro900 because the mod doesn´t really help my problem with the highs, I can live with the amount of bass, but the Ultrasone highs are too unnatural for me.
I have tried to enhance the sound by adding a Silverdragon cable (I bought from Kees) but it didn´t help the highs, it only tightened up the bass.
The HD650 is a very good headphone I would never sell....
I´ve heard the Grado RS1 and 2 and I didn´t like those either (again the treble was too much for me)
...hope that helps?
Maybe it's just that the two doesn't got good synergy together.
I've used Pro 900 with a 6p13p tube amplifier, and it's good combination.
But well, the amplifier does smooth down the treble a lot.