Audio Research SP9 family

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.

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.

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.