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. 
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.
I've always liked the Audio Research house sound, and looking into the 10 pre - reading this thread has been really illuminating. Thanks to everyone for your impressions.

I remember listening to an Audio Research amp and pre for the first time, literally a day after they arrived at my local dealer, who had just started carrying the line.

I was less than impressed. The treble would occasionally leap up and bite, and the overall effect was wooly in the lower midband and bass. I left, surprised that they would charge that much for the pair, as frankly my own less pricey tube gear sounded far better.

Fast forward six months later, and I returned to the same dealer - and those ARC boxes were playing in the same room. They sounded completely different - absolutely beautiful, with an extended and sweet treble, and a new clarity to the mids and bass. I couldn't stop listening for at least 45 minutes - they were hooked up to Wilson Sabrinas and sounded perfect, without deviation, the entire time.

I asked a staff member what happened, and he smiled nonchalantly. "Oh, they just finally broke in - all the ARC stuff does that, to varying degrees." He told me to prepare for a good 500 hours of break-in time, and they will reward your patience. That is so true for this particular line - it is a profound night and day difference.