I've tried now three different examples of the UMC device. My standard test is to connect output to input and run the RMAA/Audiotester/whatever test. With my Lexicon USB I-Onix U22 device I get very low distortion with output connected to input, and levels set to give about -1dBFS on the record input. It varies slightly with output level and input gain, but generally, it's well below 0.01% distortion, so low enough for Jazz.
With the UMC devices, I got something around 0.5% distortion on a back-to back test, and which varied a lot depending on the output level and input gain. It seems that the ADC part is the problem, not the DAC, although I couldn't get anywhere near Amir's figures on distortion of the DAC, nevertheless, DAC distortions were much lower than the ADC. I use a UMC at our radio station for Outside Broadcasts, where the ADC path is used only for cueing, so non-critical, but I wouldn't want to use the ADC for anything important, like making recordings.
S.