I don't feel like reading through the many pages here, but after reading your first post, I felt that I had to reply. Driving a headphone properly is more than just volume. Even my K240, which are a great first headphone unamped, DEFINITELY benefit from an amplifier. My Ultrasone Pro 650, like your 550, sound great unamped. However, while they work unamped, you aren't going to be at their full potential until you add an amplifier. If you wanted to run your headphones unamped and be impressed, you should have gotten the hfi-580 or DJ1 Pro. Don't blame the headphones for your incorrect use. /end rant
AKG may run at 32 ohms, but they take WAY WAY WAY more current than an ultrasone to run properly.