I recently bought the SRH750DJ - still waiting to receive shipment. It's currently sold new through a seller on Amazon.com for $104 shipped - a very good price. I don't consider these headphones to be "sleek" or portable looking, but it is known for being efficient. I expect its bass to impress with additional amplification. But I have no idea how it would do from an iphone.
Yeah I've got mine coming too, should be here in the next week if not tomorrow. Doubt the bass will be punchier then the XB500's but I'm sure the texture will impress as well as the rest of frequencies.