Does the Rio Central have a "sleep" mode, or do you have to turn it on each time you want to use a rio receiver, or do you just leave it on all of the time?

Yes, the Rio Central does have a stanby mode, very similar to the empeg's standby mode. It is meant to be left powered on all the time, and placed into standby when not in use.

In fact, there is no way to power it off from the front panel or the remote. You can only put it into standby. On the back of my prototype (in a hard to reach spot) there is a master power switch that is the equivalent of unplugging the power, but I don't know whether the final units even included that at all.

It serves the Rio Receivers equally well whether it's in standby mode or awake and playing music itself. It also can serve receivers and play music while it's ripping and encoding CDs.