I just bought a used OpticalRendu. I had 3 before sold 2 and just got one more yesterday. The Fibre Optical just before the DAC (or better yet into the DAC) sounds better to me. I would assume that the UltraRendu would also be lower in the hierarchy to my ears.
I have compared the OpticalRendu to the PlaybBack Designs Stream-IF and the Lumin X1 fibre. THe Stream-IF was compared with the SPDIF since I do not own the Playback Designs DAC to compare to the Fibre Optical input. My feeling is that the PB streamer will be the one I think as the best.
The OpticalRendu is as good as the Lumin X1 streamer. The Rendu is quieter which makes it seem more refined. Neither is better than the other for my ears.
Sonore is replacing the OpticalRendu this month so there should be more used units coming up for sale.