If you get enough volume and there's no distortion then they are being supplied with enough power. Having the knob at 3 o'clock shouldn't be a problem.
It's quite normal to experience the sound being "distant" when moving from closed headphones to open ones. One week should be more than enough time to adapt though. How long have you listened to the K702 before going back to the ATH-M50?
I have them both connected to the Bechnmark's twin headphone outs. Maybe upgrading to the dedicated amplifier will give a good jump in sound quality? Although some people say that HPA2 is already state of the art.