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

Sometimes, AI can be your friend:

An integrated amplifier increases the strength of electrical signals to reproduce sound.  It combines a preamp, power amp, and other functions into a single unit. 

IOW, an integrated amplifier already includes a preamp so, no, if you have an integrated amplifier you should not need a stand-alone preamp.

Back in the day, preamps amplified very small electrical signals so they could then be adequately amplified by the power amplifier.  When digital players and DACs began outputting higher voltage signals, some people began using them to drive power amplifiers directly.  In those cases, the only other thing needed was a method to adjust the volume, so passive volume controls were introduced and, later, on-board volume controls began to show up in DACs.

However, some listeners began to notice that their systems sounded better with a preamp in the signal reproduction chain.  Why would a preamp be necessary when the DAC has enough output voltage to drive the power amp directly?  It turns out that having active preamp circuitry, even in cases where the DAC output offers plenty of voltage, improves the impedance ratio between the output from the DAC to the input into the amplifier, which reduces the effects of the interconnect cables.  As with most things in audio, there are trade-offs and the success of getting good sound without a preamp depends on the length of cables, the specific equipment used, as well as listener preferences.  Therefore, one size does not fit all. 

Having enough output voltage from a source component to drive the amplifier(s) is a good start, but may not be enough for some listeners depending on their equipment, set-up configuration, and listening preferences. Listeners who use passive volume controls, either inside or outside of the DAC, typically value the clarity and absence of noise they perceive from removing the active stage from their signal path. However, even with source components having enough voltage to drive the amplifiers, impedance differences between source and amplifier, and cable length, can affect the sound. Improved dynamics, body, and tonal qualities are what I read about most from folks who switch from using passive volume controls to an active stage, or a transformer/autoformer based solution.

Of course, active circuitry and transformers add their own sonic changes (and potentially noise) to the signal so, the trade-offs are sometimes described as improved clarity and lower noise when using a passive solution vs. improved body and energy when using active circuitry.  Choices include either:

• Live with the impedance relationship between the source component (usually a DAC) and the amplifier(s) and minimize the effects of impedance by keeping IC cables short and selecting equipment with low output impedance and high input impedance, or
• Utilize an active stage (or transformers/autoformers) between the source and amplifier(s) in order to improve the output/input impedance relationship.

In cases where the source output voltage is sufficient, the active stage can be as simple as a unity-gain buffer (you would not need more gain), or transformers/autoformers, which can be set up for unity or other levels of gain up to about 6dB. Examples include totaldac’s d1-driver-sublime, which I believe is basically a low (or unity) gain active stage, or Empirical Audio’s Final Drive, which are a pair of passive transformer buffers/selectors. In both examples, the manufacturer’s goal was to insert a device in-line that optimizes impedance matching in order to improve the sound of their DAC when directly driving amplifier(s).