In that case you don't need a preamp. You just need a DAC with sufficient output voltage (depending on the sensitivity/gain of the power amp). Most DACs can drive most home power amps. "Professional" power amps with balanced XLR inputs sometimes have less gain so they need higher output from the DAC (or preamp).
You might want a physical volume control knob or remote and you may also want a headphone output which costs the manufacturer very little to add.
Traditionally, a preamp is a "control center" with multiple-selectable inputs, volume control and tone controls and frequently other switches & controls. A preamp has a bit of available gain-amplification but most-often it's attenuating the signal rather than amplifying... in the old days it included a phono preamp which is truly a "preamp" with lots of gain and some modern preamps include a phono input. (A microphone preamp is also a true preamp with lots of gain.)
An integrated amp adds a power amp section.
A receiver adds a tuner, and since stereo receivers and audio video receivers are mass produced and sold into a competitive market they are usually more economical than an integrated amp. An AVR includes a surround decoder and DAC, and multiple power amplifier channels.
