ClownBurner, although the frequency response of the Radio Shack isolator fell off at the lower frequencies, the most damaging effect was a marked increase in distortion below about 100Hz. This wouldn't show up on your RTA, but would on your ears. I didn't bench test the PAC unit, but would suggest either measuring it's low frequency distortion or giving it a critical A/B listenening test in the low frequencies.

Using the AUX in on the Becker was a headache, but extreme grounding between the Becker and Empeg chassis or a DC-DC converter can make it work without resorting to an isolator.

I used a 12V converter from an old laptop car adapter. If you go this route, make sure you get one that shows a voltage input range below 10V (and up to at least 16V) to indicate that it's really a DC-DC inverter. Otherwise, you can buy general purpose inverters from places like Jameco.com.