Thanks! I am using Topping D30 + A30 for now. It's transparent and doesn't add much color to the sound. Don't think I am going to upgrade AMP any soon, maybe change some op-amps on A30 as past time. I do realize AMP can make sound a bit different, especially comparing sound from iPhone and dedicated DAC/AMP. However, I am not trying to make HD600 sound as perfect as possible. I already have DT1990 for that. And to be honest, I do enjoy how HD600 sounds straight from my iphone.