Tubes can easily take 100 hours to burn in and start sounding like they will for most of their life. Also, don't even consider any critical listening for the first 20 minutes the unit is on. I have found with the two tube preamps I have owned that the major difference is going to be in sound stage. You should get a wider and deeper soundstage. You NOS RCAs will likely contribute to this further. If they are the RCAs I am thinking of, you aren't going to hear a lot of "warming" but should get a bit more air and especially depth in terms of staging. You might also need to tweak your speaker placement a hair, especially in terms of how they are towed in. I found moving from a SS to a tube unit I had to alter my towing about an inch.
One thing to consider is to take notes on 3 or 4 songs you know well with each preamp. Your memory, even a few minutes apart for listening sessions, is inadequate. I have heard radical differences between speakers where sounds just go away but it is tough to pinpoint it strictly from memory. When music is being played your mind is processing an infinite number of sounds and trying to recall differences is very hard.
One thing to consider is to take notes on 3 or 4 songs you know well with each preamp. Your memory, even a few minutes apart for listening sessions, is inadequate. I have heard radical differences between speakers where sounds just go away but it is tough to pinpoint it strictly from memory. When music is being played your mind is processing an infinite number of sounds and trying to recall differences is very hard.