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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i am still running my 5se, 5 k happy listening hours later…..put HRS Nimbus couplers under….

lovely….

Any recommendation on wether to get the ref 6 to 6 SE upgrade for $3k or trade in the old ref 6 and get a new 6SE for $5500?