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

Some people like a preamp even with an integrated because they want to use room correction like DIRAC

I have a luxman L-507z integrated amp. I havent seen anyone put a tube pre-amp on it yet. Altho with cornwalls it might be interesting. 

Mulviling and ghdprentice hit the nail on the head. A good preamp is so important. I don't mind a little coloration but a nice preamp adds body and texture. I've tried a couple times without a pre and the music was analytical and boring. I don't care what anyone says there is nothing like a good preamp in your system.  A former friend of mine argued until he was blue I was wrong.  I kept laughing and said you will not convince me because I know better.  

I wish I could remember who wrote a review on preamps in a system but they unequivocally stress they are very beneficial.  

My understanding is that source components with variable outputs typically have a higher impedance and lower maximum voltage than would be the case with a good preamp or line stage, so the latter “drive” the amp input stage with more authority, not only relating to the volume level, but dynamics and the ability to generate an expansive sound image…source material permitting.  Very few integrated amps put the same attention to parts and power supply as separate preamps have.  That said, I am in the process of consolidating a system with a separate line stage (C-J PF-R), phono preamp (Phono Box DS-3), and headphone amp (Head Box S2) into a new one that does all 3 in 1…the new Quad 33.

I think it is pretty obvious why an integrated amp, in most cases, is not going to sound as good as separates. I know of several people that drive their tube integrated amps with a separate tube preamp. In my experience the preamp section rarely matches the quality of the amp section or vice versa. The only reasons to go with an integrated would be size, cost or simplicity.