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

Take any solid state integrated and hook a really good tube preamp to it and you’ll have your answer.  

Some people prefer separates, some people prefer integrated. No hard and fast rule. 

If you have an integrated amp a separate preamp/line stage is not needed. 

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.