Yup, I have it on second highest gain setting for now (64-300 ohms, HD650s are 300 ohms). The highest setting (300-600 ohms) gives it a little more power, but the bass is a bit distorted, so I think the middle setting sounds best. In either case, they are damned loud - when listening through ASIO in Foobar, I have to set the gain way way down, basically just working the left-most portion of the volume bar. As for the op-amps, are the stock said to be not very good? I may be willing to experiment in the future, but I'm pretty happy with the sound I have right now with my Senns.