Ken, you obviously have a great system. Since you are recording to an Otari 5050 and playing back the LPs that you recorded with the 5050, I am surprised that they sound identical. I would think the LP would sound better played straight through your system based on your CAT preamp getting a signal straight from the LP vice playing back a SS signal from the 5050.
Steve, you say that difference=distortion, but in this case I am not so sure. When one thinks of distortion in a high fidelity system, that should be a negative and not a positive. The sound should become worse and lose fidelity to the master. If you upgrade your preamp, or cartridge, or amp, etc., and you hear a difference compared to what you used to hear and you think your system has made a big improvement because of your new investment, is that distortion? I know that what I am proposing is somewhat different in that we are making a copy of something onto tape and playing it back over the "same" system, except in this case when I listen to an LP straight-up, the Ampex 350s are not in that play back chain.
JV from TAS once made an analogy that compared a snapshot of music to paint-by-numbers painting. I liked his analogy. He was using the analogy to compare SS gear to tubes. His analogy was that SS put enough colors on enough numbers that you could make out what the painting was and that tubes filled in all of the blank numbers with paint so that the painting was completely fleshed out. I think that what I am hearing is close to JV's analogy in that when the LP is converted to tape and played back through the Ampex 350s, there is now much more information presented. There are way more numbers that now have paint on them and the painting is much easier to understand than before when you didn't realize that you weren't seeing the complete picture. One could also argue that maybe the Ampex 350s are coloring outside of the lines so to speak which would bolster your argument. I don't think so, but maybe I will change my mind with time.
In the meantime, I wish someone else with the appropriate gear could perform this experiment and tell us what they think. The key to this experiment is playing back the tape through tube repros into your preamp. I have already heard the SS comparison and it doesn't work. My brother is a musician with a set of very sharp ears (he does not consider himself an audiophile) and he is grinning from ear-to-ear when he listens to the tapes I made him. He for one is convinced that that tapes he is hearing sound superior to the LPs they came from (and he was present when I made some of these tapes for him) and of course being my brother, I never hear the end of that. If you told him that the difference he was hearing was due to distortion, I know he would laugh and question your ability to hear what music really sounds like.
Another experiment that I would like to perform is to make the Ampex 350s part of my LP playback chain and see if the same magic is there. Unfortunately, the output from my cartridge is .23mv and I just don't know how I could incorporate the 350s with my SA-2 and SA-5.1.