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

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. 

@paqua123 - The Luxman integrateds really are something special.  Many manufacturers claim that their integrateds have their top of the line preamp circuits built in and who knows but in the case of the Luxmans I honestly believe it.   The difficult thing for me is that I'd actually like to go fully active, and trade from the Luxman integrated to Luxman pre, but the pre's are like 2x as expensive as the integrateds.

Choice and sound quality. You don’t "need" a preamp.

A good example of having separates is an electrostatic speaker. You love the sound of tubes, but the tube amps tend to be poor performers (queue the chorus of A’goners who will "not always.." this post) so you mix a tube pre wiht solid state amp.

Also, simply power. Having separates allows you to up/downgrade your power size and try different amps (D, A, A/B) without changing the pre.

In a sense, it's similar to having a separate DAC from your preamp.  Having these separate lets you change your DAC while keeping everything else.