esssseye,

Yeah, the Sony XA39 series is nowhere to be found. Of course, one sold on EBay for $5 just a few days before I knew it ever existed. So, keep your eyes on EBay for it.

Optionally, you can get the somewhat shady PIE MPSS-4 like I did. I am happy with it controling my empeg and Alpine HU. Though I can't explain the wiring (since my installer did it), here's my take on how it works:

The MPSS-4 allows for a primary and secondary audio source.
The primary is always powered on. When the primary is sending an audio signal, the secondary unit is powered off and the MPSS-4 sends the primary's signal to the amp.
When the primary is mute (or something like it stops a signal on the amp sense wire or something), the secondary unit powers on, the primary unit stays powered on, and te MPSS-4 sends the secondary's signal to the amp.

Please note, since I didn't do the install, I may have "primary" and "secondary" switched in the above explanation. However, the above explanation is the logic for the MPSS-4.

So yeah, why DOES it power down the secondary unit? Who knows, but eventually you learn to deal. But because of this power-off deal, you'll want to make sure your empeg is on the always-powered-on line (primary, I'm guessing). That's the way mine is done. empeg powers up and sends signal to the amp. To switch, I just put the empeg into standby (by holding the top button) and my Alpine HU powers on and sends signal to the amp. The empeg stays in standby. If I touch any button on the empeg, the signal IMMEDIATELY switches back to the empeg and the second HU powers off. So, yeah, it takes a second or two to switch from empeg to radio, but it's instant when going back to the empeg.

I hope this helps. If you have any other questions, please ask. If you want, I can even scan the MPSS-4 install sheet(s) and post them (if that's legal). My installers had no friggin clue about the empeg or the MPSS-4 but they managed to install it all perfectly with those directions. Good luck!