A friend of mine told me that I will see a huge difference in sound quality once the tube preamp breaks in. I was wondering if I should get a break in cd or just wait till normal listening does the job.
Assuming the tube preamp does not contain significant digital circuitry, whether it is playing music or simply powered up and playing nothing makes very little difference in the amount of current conducted by the tubes and pretty much everything else in it. So I would expect that simply leaving it on would break it in as effectively as normal listening. (That is not necessarily true for most power amps and many digital components, btw).
I have no opinion about breakin CDs, though, as I’ve never used one and I have no particular knowledge of the technical characteristics of the signals they provide.
Also, given that the power amp and the speakers are "slightly used," and might have sat unused for much of the time they were in the dealer’s shop, I wouldn’t rule out the possibility that their breakin state might be more of a factor in what you are perceiving than the preamp’s.
Good luck. Regards,
-- Al