I know this may not be a popular opinion but essentially all pre-amps will sound the same if they are doing their job well. They are supposed to simply transmit sound and not modify (with the exception of DSP modes)it in any way. So I would say, the differentiating factor would be the features and build quality. If you have no need for the features I would highly doubt you would be able to hear a sound difference between the 950 and 990 and any other pre-amp in an A/B comparison. It could be feasible that one would be more electronically perfect than the other but if the variances are below human hearing who cares?? Modern electronics have such a low noise floor, and virtually linear frequency response that nowadays our focus should be on the real things that cause huge differences in sound: your speakers and your listening room. Dont get me wrong, I love the 990 but if they were both still on sale I would get the 950 since I dont need the 990 feature set. And I am sure they would both sound JUST AS GOOD.