Wait a minute, Jon. Are you trying to tell us that once the fundamental design elements are accounted for, that throwing some
Reliable Exotica Polypropylene & Copper Foil caps in the signal path isn't going to turn water into gold? Why that's just rubbish and defies all taperssection logic. How dare you challenge our audiophile beliefs!
/sarcasm
I really love these kinds of debates and I'm sure (hoping, actually) that this thread will spiral out of control.
IMO, it would behoove us all to idealize preamps as being sonically equivalent if/when their performance specifications are the same. A preamp should have one job; to amply the input equally at all frequencies (somewhat of a generalization). They shouldn't impart a 'flavor,' otherwise they are doing more than just amplification and thus, not an ideal preamp. If you want to adjust flavors, there are far better means to do so, namely using different microphones or post-processing.
There's a lot of snake-oil in the preamp business and
far too much emphasis placed on their sonic importance in the signal chain. I own
a lot of preamps... far more than what is reasonable. I also build just as many on my own, but I learned a long time ago that accumulating them to have a 'box of crayons' as the metaphor goes was the wrong reason. As Jon states, I'm not able to predict which 'flavor' would be best in a given situation and even if I could, there are numerous other factors that play a much larger role in achieving 'ultimate quality.' Over the years, I've learned that as long as a preamp's specifications are 'reasonably close' to ideal specifications, features such as powering efficiency or source options, inputs/outputs, form factor, metering, etc. are all far more important than "how does this preamp sound."
As to the original question, when there are transformers involved sonic testing can be important, but I believe that all three of those units use the same Lundahl transformers. There might be some differences, but these units are now too old for me to remember or care to research. In short, transformers are not an ideal component in the signal path and create distortion / artifacts. Sometimes this can pleasing to the ear, but transformers are prohibitively expensive components to audition, often costing $100 or more per unit, and the design considerations for why they used in the past are of far less importance today. In essence, transformers have been and always will be a non-ideal workaround.
YMMV