The thing with the U-Retro though is, while it is an awesome preamp it is also very coloured and when installed you sacrifce the natural sound of your bass for the U-Retro sound - if you are lucky it will enhance the sound you require, if not it will take it over. I installed one in a high spec J bass recently and it only lasted 2 rehearsals (the second time was to give it the benefit of the doubt and also to work on my theory as to why it didn't work for me). Soloed it was utterly fantastic but in a band situation it somehow killed the life in the room - my theory is that because of the powerful and broad spectrum of frequencies it produces it clashed with the other frequencies from the band and cancelled them out....for everyone.
The Schack preamp is about the best I've ever come across, easily rivalling the JE for spectrum but is also more customisable (internal micro switches allow you fine tune it to your bass or taste) and more importantly for the OP will do complete passive control. Try finding one though.