I've given that ZZ Top album a good listen, and there are some surprisingly clean bass tracks on there! But for the dirty songs, I still think it sounds very close to the EBS Billy (not that I expect for a minute that Dusty Hill would have used one!).
I've been fiddling with mine and it really helps if I run my cheap Behringer EQ in the dirty loop to pull out the lows, it really tightens up the distortion and doesn't interfere with the clean lows.
That said, on the ZZ Top recording, the tone of the bass is doing most of the work - the distortion is the seasoning.