Audio Research Ref 10 vs 40th Anniversary preamp

As barrysandy said above... If you compare the 40 and 10 side-by-side, the circuits are identical down to every trace and part.  The tube choices don't really matter much because you can easily put your favorite 6550 in the power supply and 6H30s in the audio circuit (I like NOS for both).  The different output coupling caps could explain some of the sonic difference, but there may be other improvements brought about by changes in wire, connectors, circuit board material, or other things that are hard to see.   

The Ref 40 and 10 are much more similar to each other than they are to the Ref 6 or it's predecessors due to the larger, more sophisticated, power supplies and complete dual-mono circuits throughout.  The 40 and the 10 have separate transformers for each channel, separate sets of regulator tubes per channel and separate 6H30 tubes for each phase of the balanced signal.  The Ref 6 shares transformers and power supply tubes between channels and shares 6H30 tubes between + and - phases of the balanced signal path.  It also shares one chassis and one main board for both power supply and audio circuit.

I own a 40 and haven't heard the 10 in my own system to compare directly, but I did have a Ref 6 for a few weeks and it's a great sounding preamp.  It's not as complete, 3-dimensional or effortless sounding as my 40 but it did offer a bit more transparency at high frequencies.  My understanding is that the 10 sounds like the 40 with some additional transparency. 

@folkfreak i bet it sounds wonderful:-)

All the mention ARC preamps track tube hours, I keep a log when as advised I roll in replacement, the the NOS Tung Sol 6550 as recommended by Andy at Vintage tube in Michigan is a spectacular upgrade to the 5se at least in my system.

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.