Audio Research SP9 family

It's been a while since I owned both the MK 2 and Mk 3.  The 3 had gold plated socket pins and I believe both the 2 and 3 had upgraded caps over the previous model.  I thought the 2 sounded better with ECC88 Tele's and the 3 with the E88CC.  Both were non aggressive on top, soft on the bottom and pleasant and musical rather than strictly accurate.  The 3 was much more detailed.  Imaging and realism were not their strong points. Of the two the 3 was the one to get.  

Generally, I don't like to criticize components, but the MkIII I purchased many years ago was without a doubt the most disappointing audio component I've experienced in over more than thirty years as audiophile. Low resolution, transparency, just a boring piece of equipment, hybrid of tubes and SS didn't make it here. Also add, I tried different tubes, isolation devices, even installed IEC for aftermarket power cords, nothing helped.

Each mod version is presumably better. How much better is impossible to say, but even the base model is a very nice preamp.. 6922 is a 6dj8 variation and said to be a better version—at least in theory, but its easy to silkscreen anything on a tube, so I would not put too much weight on that. Either tube works. What ARC did at any given time may have just reflected their part sourcing situation.