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. 
Well guys, I am back again, ... 1 year and a few months later, ... with some comments about the ARC Ref 6SE and the significance of break-in.  So, I finally swapped out my Ref 6 for the SE upgrade in the Fall of 2020.  I am happy to report that I am *very* pleasantly surprised and am *very*pleased. 

Right out of the box, ... the Ref 6SE sounded off.  Hard to describe. 

However, after 100 hours, I could start to hear real improvements in SQ.  The Ref 6SE sounded faster, better focus, better sound-stage, better imaging.  Bass was tighter too.  I recall listening to some jazz records and immediately noticed how tight the bass instrument sounded. Drum thwacks and cymbals ... really tight and clear. 

And all of these SQ improvements continued as I racked up more hours on the unit.  I have about 500 hours on my Ref 6SE now and it is pretty well broken in.  

As to bwright's comments and observations above, ... he is spot on.  Brand new ARC gears needs to break-in before one can really appreciate its quality and attributes.  In many ways, buying pre-owned ARC gear has an advantage:  the pre-owned unit should be broken and any kinks worked out.

It only shows ARC was selling a flawed and veiled Ref 6 before the SE upgrade...

And charging you 10k for that « flawed » non SE pre...
Respectfully, techno_dude,... I do not agree with your comment.  The Ref 6 is a wonderful preamp.  The SE upgrade made the Reg 6 sound better.  I understand that the changes consisted mostly of new caps and wire.  

ARC is constantly trying to improve its product line.  If upgrades to one component can be transferred to another product, ARC oftentimes will do that.  For example, ARC made SE improved its Ref 150 amp, the Ref 6 preamp and the Ref Phono 3.  

I own the Ref 150.  ARC did the SE upgrade several years ago.  The upgrade included changing out the KT-120 tubes for KT-150 tubes, plus other changes.  I can tell you that the SE upgrade significantly improved the SQ of my Ref 150.  Ditto re the Ref 6, which I used to own but had upgraded to the SE version.

Anyway, that is my opinion.