So, here's the deal with flats+rounds. I realized that I really play my bass like two separate instruments:
1) Holding down the bottom
2) top-end zing, more like a guitar
The flats just had too muted a high end in the upper registers. If you'll forgive the pun, they were a little flat.
So, I have a smooth rolloff in the lower ranges, and bright, zingy top end. I don't know if that makes any sense. I think it's also related to the fact that I have a pretty hifi rig, so the flats still have presence, and aren't all thud.
This is a bit like dancing about architecture. If you make it to Seattle, you're welcome to play it.