If you're looking into dual head unit with external amp, consider the "source switching relay" by Precision Instrument Electronics (PIE) MPSS-4. It's an alternative to the Sony XA-39 which I found to not be available (back when I looked).

The MPSS-4 takes a primary and secondary head unit. When the primary is sending audio signal, the secondary is powered down and the primary's sound goes to the amp. The secondary is powered on when the primary stops sending signal, but the primary is left powered on.

This works GREAT for the empeg, I've found (contrary to my initial opinion of it). Empeg = primary. When the empeg is playing, the secondary (CD player, or whatever) is powered off. When you put the Empeg into standby, it remains powered on and the secondary is powered on and sends signal to the external amp. To turn the empeg back on, simply press any button on the empeg to wake it from standby. BOOM, instant empeg sound again.

So, the empeg never gets hard-powered-off, probably causing less strain to it. Ah, let the secondary power off when you aren't using it. Less strain on the car battery? Maybe.

Best of all, if you're going somewhere that you can't bring your empeg out of the car, just leave it at home. You'll still have radio.