"The job of a preamplifier is to turn a low-voltage, high-impedance signal susceptible to signal degradation, into a higher-voltage, low-impedance signal. This process is often referred to as adding gain, and preamplifier specifications often refer to how much gain the preamp provides without introducing audible noise. The output of the preamplifier can then be safely sent along to a power amplifier that will raise the current to levels that can drive speakers.
In the case, a phonograph does not actually contain a signal at the correct frequency response for playback systems. They are recorded differently to reduce audible artifacts. This means that you must have a phono preamp somewhere in your signal chain in order to play back a vinyl record. This is a special type of preamp that applies RIAA equalization"
Usually, the pre-amplifier is more closer to the user; the audio power amplifier is close to speaker.
So the convinient way for the user to adjust the volume is to control the volume either on audio source, or on the pre-amplifier
You can test your product and compare with the follwing RIAA equolization curve.
The RIAA equalization curve for playback of vinyl records. The recording curve performs the inverse function, reducing low frequencies and boosting high frequencies.