Congratulations on getting an ET3 SE.
It is the models with teflon capacitors, like your ET3 SE, that people claim need a long break in. I don't have the ET3 SE but I have a current model CJ preamp with teflon capacitors that I bought used - though it had not been used for a while (sitting in a box). Although it had done about 1300 hours before I got it, for me it took about 90 hours of music playing through it to sound consistent. Kind of a re-break in. Until then it had some bad days, being fatiguing. I would have no doubt that a new one could need 300-500 to fully break in based on reports I have read. There are various threads here on audioaficionado that discuss this.
Since your unit is a demo unit - won't it already be broken in? Your dealer might have done all the hard work for you and you can just sit back and enjoy it.
As our preamps are not the same model, I'll leave it to others to report about tubes they have used. You might want to check through some of the old threads in the
conrad johnson area of audioaficionado as there is quite a bit of info on tubes that people recommend.
Anthony