I agree. And that is exactly what a blind ABX test is: It is listening . . . without allowing preconceived bias to affect your perception of what you are hearing. It is always interesting to me how those who tout their abilities to hear minute variations between components in their systems over a long period of time always seem to be the ones who are most vociferous about the inadequacy of blind testing. I guess listening to music is more about art than science. But, to each his own.
I disagree that you can't tell a difference between preamps using ABX testing. It is certainly easier to tell differences between some than others. Two well-designed neutral-sounding preamps will be difficult to tell apart. I was able to tell the difference between my Sanders Sound Preamp and my ARC Ref 3 preamp 75% of the times I tried. Both are very neutral preamps. The differences were incredibly subtle and it was difficult, but I would say that is a statistically significant percentage to tell one from the other. However, I was just doing blind A/B testing, and not A/B/X.
I think what you will find with a tube preamp vs. your Rotel is a clearer, less grainy presentation with a much smoother midrange. You will notice less etch in the highs. You may notice less articulate bass, depending on the preamp you choose. And that really is the key. Some tube preamps will be easy to tell from your Rotel because they add so much of their own sound. Others, that are more neutral, may be tougher to distinguish. But if your system is capable of resolving the differences, you should be able to hear them in a blind test.
The question is: What type of improvement in sound are you looking for in substituting a tube preamp? Then you can choose which preamps to consider based on those parameters. I encourage anyone to try a double blind test just to see for themselves how subtle the differences are between quality components. Then you have to ask yourself, is it worth thousands of dollars to upgrade that component for a few subtle differences in presentation. For the higher-end systems and those with money to spend, the answer is a definite yes. For others, maybe not so much.