Dumb question......why do you need a preamp?

The way your question is worded, the correct answer may be, you don't need a preamp as it is already incorporated into your integrated amp.  

The answer from @soix above, suggests the preamp section of an integrated amp is not sufficient, and adding a separate tube preamp will improve the sound.

All further discussion will be subjective opinions on who makes the best preamp. 🤣   Cheers.

@erik_squires  I agree with your statement.  I run SoundLab Majestic 745 speakers.  For power I use the Hegel H30 SS mono blocks.  I had the matching Hegel SS preamp for a while but found a significant sound improvement (IMHO) when I introduced the BAT Rex 3 Line Stage tube preamp.  In my application, it's the best of both worlds.  Cheers.