Hi; I upgraded over the years from an SP9 to SP9MKII to SP11 to REF 3 to REF 5SE to REF 6.
Loved the SP9 MK II and with new tubes there was not a jaw dropping difference between that and the REF 3.
Compared the REF 3 to a REF 5 and there wasn't enough of a difference to me to justify going to the REF 5.
However, in direct comparison between the REF 3 and the REF 5 SE, there was a large difference. Don't get me wrong. To this day, the REF 3 is still one of the best out there. it is very nice indeed. However, the REF 5SE was simply nicer to me.
I also did a direct A/B comparison between the REF 5SE and the REF 6 and to me there was not a jaw dropping difference. The REF 6 was slightly better than the REF 5SE.
Therefore, it comes down to budget. I was going to keep the REF 5SE, but, I bought it at a very nice price and then learned about the Audio Research trade in-trade up policy at the time and they gave me 75% of MSR for the REF 5SE. They basically gave me more for the REF 5SE than what I paid for it. It made going up to the REF 6 a very good economical decision.
No regrets. it is very hard indeed to beat the REF 6. The REF 10 is the standard bearer. But, if you can afford it, you won't regret the REF 6. That said, you also won't regret getting a REF 5SE either.
enjoy