OP,
I went from a REF5SE to REF6SE so, no direct correlation.
However, I have heard long term the REF 3 phono stage and the REF 3SE Phonostage. I believe the internal changes we similar if not exactly the same as would be for the preamp (I think the two are nearly the same unit overall). The change was subtle but important. Greater detail and lower noise and higher dynamics. I would do it… as opposed to waiting for the REF 7.