Well, for now, I've re-wired again and have the empeg's output plugged into one of those tape adapter thingies. It's kinda ugly to have a wire sticking out of my tape deck and it's notably lesser quality (as should be expected).

At half volume, I don't notice any distortion (with the empeg direct to the bose amp half volume was all I had anyway). So now I have the option of upping the volume more at the cost of a little distortion.

The plus is that I can use my steering wheel remote for volume again (factor remote for Bose HU, just select tape to hear empeg). And for the rare times I play CDs, they play at full volume without distortion (as they did before I added the empeg).

If I were a true audiophile, I'd go buy a real amp and get rid of the Bose one.... maybe next year. The patchwork job I've got now should tide me over.