Here's my report from two days of testing with four people, which includes myself: My initial impressions were that the Ref6SE was more laid back sounding and slightly smoother especially in the mids versus the Ref6, which had a more forward presentation in the mids. However this was comparing 6n6p tubes in the Ref6 against the factory tubes in the Ref6SE.
I tested with Person A on the next day, and he liked the Ref6SE better in every test and had no idea which unit was which. He does not have an audiophile background, but played several instruments and was in the radio industry for quite some time. He described the Ref6SE as "clearer."
Person B and C had the same thoughts as each other and were also tested blind on the same day as Person A but in a separate session. They thought the Ref6 with the factory tubes sounded the best. Unlike Person A, Persons B and C thought the Ref6 was more transparent, but that the Ref6SE had more density. They thought the Ref6SE sounded more tubey. Interestingly, they suspected that they liked the Ref6SE better with the 6n6p tubes and the Ref6 better with the factory tubes, which I ended up testing after they left, but couldn't really notice any differences probably due to fatigue at that point.
Ultimately, we all agreed that the upgrade was not worth the cost, and I was not even sure if I could distinguish between the two if blind-tested once we used the same tubes. Ideally, I like to test blind and sighted with short A/B comparisons and by listening to full albums and on multiple days and over a long period, but since this was a dealer demo, unfortunately, we didn't have the luxury of doing that. I will also say that I know that components can yield different results in different systems but just wanted to provide my experience with the two preamps in my system as a data point for others.