My last preamp upgrade was from a Reference 5SE to a Reference 6SE. So, I haven’t heard the Ref 6. The Ref 5SE vs Ref 6SE Is a nice upgrade to the same kind of sound. But the 6 versus 6SE are one of carefully chosen internal components versus design change, and all that I know who have made the upgrade have said it is well worth it. I like listening to music, as opposed to buying and breaking in components. So, I would get the SE, and not waste time on analysis, sending in for upgrades… and spend the time enjoying your system.
If you want more info, go to the audio aficionado forum. Preamps like the Ref 6/ SE are average preamps among members… there are lots of discussions on the two. The consensus is definitely worth it.