If your powered speakers have an on/off switch then the Mini-X will work fine. Unfortunately the output on the Mini-X is a straight loop-through of the input and bypasses the volume control. With my B&W MM-1 speakers this is not a problem as I can adjust the volume on the speakers. Apart from that the Mini-X remains a great amp for the HE-500. I still have no regrets about mine. There is a potential for noise on some Mini-X amps and there is also the issue of limited volume adjustment with the volume pot. You might want to have a look at the Mini-X thread to see the resistor network you can employ to lower the output and noise floor.
No, unfortunately they're passive speakers so that's no good. I'm trying to determine if there exists some sort of passive switch or pass-through component with a headphone tap that might be able to suit my needs.