I intend to connect my Mark II to an aftermarket receiver to serve as a tuner and amplifier.

I'm not sure what is the best way to supply power to my rio. Should I simply splice into the constant and switched power leads from my existing receiver, or do I need to find other power sources?

Yes, connect the constant and switched power to the same place that the other receiver's constant and switched power are connected to.

If you are using the other receiver as your main audio source (empeg goes to the other receiver's aux-ins), then do not connect the empeg's amp-remote wire to anything).