There’s no discernable difference in the basic circuit besides the microelectronics. The output film caps and the original PS 6550C tubes used are different. There’s a noticeable difference in sonics between the REF40’s SED Winged "C" 6550C and the REF10’s Sovtek WEs with the former being a little warmer and a touch more laid back and the latter emphasizing the leading edge and a little more upfront. Can’t speak to the caps, but I’m sure they’re likely the rest of the difference. I prefer the SED winged Cs, but you may like the current tube and maybe the REF10 better. I actually like the Sovteks better in the REF10 phono than the SED Winged Cs but neither are my favorite. Go figure.
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.