I love the HD650, sounds great with a uDac 2. I'm very hesitant to buy any other amps because this combo sounds quite excellent and I'm sure I'd have to drop big $$$ to beat it hands down.
Anyone ever notice/mention that the HD650 sound more clear when it is rotated slightly to the back, and pushed forward slightly on the head? It's easy, comfortable even, to wear them in a position that actually slightly muffles the high end. The inside of the ear pad should touch or even push slightly on the back of the ear for optimal sound. Wearing them too loose would keep them from staying in this position.
I used to own both the HD650 and uDac at the same time...the HD650s do get SOOO much better than Nuforce would have you believe.
A good upgrade would be say the Schiit Asgard and Bifrost. The best I've heard the HD650s is from a 3 channel B22 fed by a Bryston BDP1 DAC. <--- Isn't this place insidious.