Would you describe type of music you listen most? What other equipment you have? I suggest find a set of songs you like most and listen most. Use that set of music when you try out different pre-amp. You would be able to tell one does more details or better stage than the other. With that, finding the characteristics are more imprtant to you.
I found Conrad Johnson 16LS is great and end up with ART2. I think CJ is great for vocal music. I do not care about deep strong bass or find the stage from a orchestra. None the less, 16LS or ART2 perform above average all around.