The system can be a big factor in how bright or even good a headphone sounds. You know what the Q701 was plugged into? It's been a while since I heard the DT 770. I found Beyers do tend to have a bit of a boominess to their bass, though different systems can alleviate this problem. I prefer the AKG and Senn bass due to them having more texture and resolution. I haven't had my K612 for about a month, I'm about to get it back and a Q701 tomorrow, can't wait to see how the two compare on a higher end system.
I totally agree with you that source has a big impact on audio quality. Yes, it was plugged into my Racoon SG300 tube AMP DAC combo. It has the PSVANE 12AT7-T tube in it. I brought it over to his house with me to make sure that I could test the Q701 with my own gear. I will be using the Racoon SG300 for my new K612 Pro when it arrives as well. This is the USB AMP DAC combo here at the link below. It is a warm sounding unit, and sounds great for the price. It is about 190 bucks in Canada, but you can get it for like 160 in the US. It sucks how everything is more expensive here. I had to pay 211 bucks for the K612 Pro including shipping, and I know you people in the US can get them for about 180 shipped.
https://www.springwateraudio.com/store/index.php?main_page=product_info&products_id=96#.VBu4YJ1rY2w