I just remember that from the REF 5 to REF 5SE, to me, it wasn't enough of a sonic improvement to "justify" me purchasing the REF 5SE from the REF 5.
However, there was enough of an improvement (just enough) for me to justify going from a REF 3 (with new tubes) to a REF 5SE. I've heard the REF 3 (with new tubes), and REF 5 and REF 5SE and a REF 6 all in the same system at the same time. That was fun.
I ended up purchasing a REF 5SE first, with the theory that I was going to sell my REF 3 to help purchase the REF 5SE. Well, my Friendly neighborhood Audio Research Dealer (Optimal Enchantment) let me hear a REF 6 at the same time. So, I sold my REF 3, traded in the REF 5SE (and received 75% of MSR for it) and all that went towards the REF 6.
I've just never heard a REF 6SE yet. Don't know if it is "that much" better than a REF 6 for me to come out of pocket for $3000 for the upgrade to a REF 6SE. I'm pretty darn happy with my REF 6, so time will tell. It would be great to "borrow" a REF 6SE and do an A/B listening comparison with the REF 6.
Also, just to be clear, Audio Research absolutely did not use "inferior" parts in previous Reference series products. They used the best their Engineers came up with at that time.
enjoy