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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Its amazing how those power supply tubes impact the circuits.  I have an Aesthetix IO loaded with NOS tubes.  I put cheap Sovtek 12AX7LPS from Jim McShane in the power supply but put NOS Mullard EL34 there also.  The difference from the Mullards is not minor.  For my REF10, will use original Winged C's when the time comes.  Honestly, at the ARC Ref level, doubt one can go wrong with picking any of their preamps. 
My buddy owns the 5SE and has a dealer demo ARC 6. In his system, I have a VERY hard time telling the difference between the two pre-amps. Perhaps a longer time with each would reveal the subtleties. He loves his 5SE and was hoping for more of the same from the 6, but "so far", he cannot justify the change. His Dealer is now suggesting the Ref 10.
I had the Ref 5se and really liked it. I demoed the ref 6 from my dealer and couldn’t believe the difference in the 2 of them.  The 6 was better in every way in my system. I sold the 5 se and had to buy the 6 brand new. 
I concur with tattooedtrackman.  I am on the list for the Ref 6 SE upgrade, which I am dubious will make much of a difference.  The main changes are the power supply caps are getting switched out and some of the wires too.  I look forward to being pleasantly surprised.