Not sure of the proper answer. Many here believe perfect reproduction is the goal. I have a very high sinad dac (Schiit Modius) that goes through my Feya+ preamp into a high Sinad NC-400. I can get very transparent reproduction when wanted. However, I sometimes like to choose the tubes from the Freya+ and find that it sounds quite good to me. Maybe not perfect, but pleasant and fun, and I know I am probably adding distortion. Similarly, I sometimes sit and listen to records and find myself captivated by the experience. Clearly they are less transparent. Not sure why it connects more emotionally at times, although I heard about a study that found there was some low level of distortion that was preferred to the more transparent reproduction by the participants.
In my opinion, phono pre-amps, along with turntables and cartridges, tend to have greater variability and are therefore more subjective in character than are good DACs and amps. Some subjective comments I have read on the Cambridge complain it's somewhat flat in the mid range. Totally subjective comments, but like tubes, may represent an actual experience. As to the MM-6, for all I know it may measure better than the Duo, but won't know that unless and until someone tests it. I bought one because when I am scratching the analog itch, the subjective impression is important to me, and the review I read was very positive. If I want more transparent reproduction, I'll listen via the DAC.
My opinion generally is that if it measures bad, the manufacturer has some explaining to do. If it measures above average, then I like to have at least one person affirm that they think it sounds good. If you think anything less than double-blind tests are useless, see above, then until something measures better than the duo, your search has ended. I believe I hear something that makes this sound really good, maybe confirmation bias, but it sure doesn't sound flat or lifeless. That said, I find ASR so valuable because Amir provides the measurements that can tell you if your equipment is all hype or actually has some basic quality. See Chord's phono pre. This ability to call junk junk is a huge contribution to the hobby. Is it the only thing that matters? You'll have to answer that for yourself.