I have heard both the REF 6 and REF 10.
I own a REF 6 and would no hesitation purchase a REF 10 if budget allowed.
The REF 6 does everything quite well. If friends asked me, depending on what their budget was, I would tell them to take either a retubed REF 3, REF 5SE or REF 6 and if their system called for it and budget allowed, a REF 10.
The REF 10 is the best pre-amp I've heard and I've heard some amazing pre-amps.
Matched with REF 250SE or 750SE amps, and that part of the system will basically disappear sonically.
As I've mentioned in other posts, when you get to this level, you really aren't missing much and jaw dropping differences really don't happen. Subtle sonic differences and nuances maybe.
The real test is to A/B listen in your home system with a REF 6 and REF 10. If that can't happen, then an A/B comparison in a store's system with a REF 6 and REF 10. Changing nothing else but the pre-amp.
Another thing that I believe people do incorrectly when comparing equipment is that they don't make volume levels.
An apple to apples comparison would entail listening a the level you desire on device A, then place a test CD on at the same level and measure with a db sound level meter the db level. Then, when satisfied, remove device A and insert device B and play the test tone and adjust the volume until it matched the first. Then, listen to the same music.
I believe that many people perceive differences in sonics as superior/inferior when actually it is volume/gain levels that are the only difference. I hear things like, the highs are more pronounced. or the bass is lower. But, my first question is, did you match volume levels first?
Anyway, if you have the system, budget and money, the REF 10 is simply great. The REF 6 is high end great also. Brand new, full retail, they both are stupidly expensive. But, on the used market? you can find really good value.
enjoy