Audio Research Ref 10 vs 40th Anniversary preamp


I was wondering what the difference in the sound between the Ref 10 and the 40th ? Not much wrote about that. I btw have the Ref 6 but never heard the 10 or the 40th. 
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minorl,  please clarify.  Which unit did you trade in?  What unit did you buy?  I own a Ref 5SE and have been thinking about the Ref 6.  Just curious if ARC would do a trade up my Ref 5SE to the Ref 6 and how much they would charge.

Thanks

BIF
I currently have the Ref5SE and Ref6 and agree that the difference is fairly large.
monorl ....  I had just bought a used Ref 5SE without ever hearing a 5se,Just going by the reviews on line and posts and threads from here on AG. I had it about 2-3 weeks and really liked it alot. And then my ARC dealer let me bring home the 6 to listen to it on my system for a few days.The first time i heard a cd with the 6 on my system i was floored. It was a very big difference from the 5SE , a Wow difference on my system and my ears. I had to have it . I am just talking about me and my system and ears. I am not doubting you what you heard but only am saying what i heard. I have no regrets what so ever of selling my 5SE for a brand new 6 thats how much i loved it and wanted it after hearing it compared to the 5SE on my system so much that i didnt even want a used one. I wanted a brand new black 6 but my ARC dealer said that ARC had no black ones in stock and it had to be made for me. I could of gotten the natural color right away but i chose to wait to have the black one built as that was my first color choice. As a matter of fact im still waiting for ARC to finish it. Its been about 2 and a half weeks now , hopefully about another week or so i will have it. 
Bifwynne, I traded in the REF 5SE for a 6.  Couldn’t pass up the deal Audio Research offered.  I don’t believe the trade in/up program is still ongoing.  But when it was in effect, they gave me 75% of MSR price. 

Enjoy