Thanks to everyone here for their help, (especially Hugo, who started me down the right track), I think I've got it licked. Even at max volume, there is no alternator whine and no appreciable digital noise in the system (Woo-hoo!)

My solution: The Empeg wasn't properly grounded. Hugo's response got me thinking and I found a much more solid direct-to-the-chassis ground point for the Empeg. That eliminated alternator whine.

The digital noise required a filter, but I've tested with sweeps, sine-waves, and square-waves, and there is no appreciable drop off anywhere in the audiable range, at least on my system. I used the PAC SNI-1 ground-loop isolator and filter. I think the digital noise was caused by the Becker's poor isolation of the aux inputs.

Cheers, and good luck!!