Have you noticed any differences sound-wise when listening to your HD600 plugged in BTR7 vs when you plug them directly into your phone? I'm looking for my first amp/dac to power my hd600, and opinions are divided about how much power you need to get the best out of them. Initially I thought about getting a BTR5/7, but some say you need a more powerful amp to get the best ouf of them. (Currently looking at fiio k7)