Dingbat-
I believe you are correct. The U retro deluxe (a modified version) can handle only 2 pickups, but Sheldon performed some magic in which he uses an RMC polydrive buffer to incorporate all the piezos and then somehow linked the u-reto thru the polydrive, or vice versa.
If you look at the controls on my bass, I got the usual 4 knobs for the U retro, then it has 4 sets of switches. top 2 determine magnetic vs piezos (or both), and patch-up/down, the bottom two switches control active/passive, and front p'up/both/rear p'up in passive mode).
As for how do i like the U-retro. I like and I don't like.
I like because its very musical, and versatile, I love the slap switch- it adds that extra tick to your playing. I also like having a lot of control over the mids. My biggest beef w/ the unit is- is once you dial in a great tone, you'll be hard pressed to find it again, period. Next, I don't think its has a lot of boost, meaning turning the control knobs full doesn't provide a wide range. Third- in general I find that I can't dial in great tones on the fly, I actually have to play w/ it alot.
I have a skjold bass w/ a Modified East preamp thats looks/sounds very similar to this u-retro pre (interesting how similar to a Dingwall those Skjold bass sound

). However Pete has made some changes to the mids which has made a world of difference.
So I understand why you pulled it out of your bass.
Honestly the pickups sound so good in passive mode that I play in passive as much as possible. But i'd love to make the P-p'up sound a little fatter sometimes, and thats why i have the active. The piezos were for stricky driving my v-bass
frank