Hi Lew,
"But I must confess, there is so much information in your post that one essential element eludes me; were the MkII and the Mk III BOTH mounted in Steve Dobbins' plinths?"
Yes, both were mounted in Steve Dobbins' plinths. However, all three plinths used different materials and construction. Each was designed to reduce noise transfer from the particular motor of each to the armboard and platter of each. Steve tried many combinations.
"Also, what platter mats did you use on the Technics tables?"
The MkIII had a SAEC SS-300 with a sorbathane "like" material on top of it. The MKII had the orignal rubber mat. Since getting my MkIII, I have tried two different mat combinations. First I compared the original rubber mat to a Saec SS-300 with a Herbie's Way Excellent Mat on top of the Saec. The Saec & Herbie combination sounded muffled and dull in comparison to the rubber mat. Next, I compared the rubber mat to the Saec mat alone. I couldn't hear any difference between these two mats. For each of these comparisons, I adjusted the VTA on my arm to get the best sound for each combination.
The 301's I heard previous to listening to Steve's 301 were uninvolving for me, no "juice". The plinths were different so maybe that would account for what I heard, I don't know for sure.
I don't remember all the modifications that Steve did to these three tables, but I know what I heard. In this situation, the MkIII was much better than the MkII but maybe in a situation with different plinths with different modificaitons the outcome may be different. I can only give you the conditions for this comparison and my opinion, for what it's worth, given these conditions.
Best,
John