You can safely ignore any references to frequency response in any preamp you buy. If it's a good quality unit, the frequency response will be more than adequate for the job. If it's a low end unit, any claims about frequency response will likely be bullshit.
The frequency range of human hearing, as mentioned above, is considered to be 20Hz to 20,000 Hz at best (measured in young girls), and in most of us is considerably worse. Although there is controversy regarding the sonic effects on human hearing of frequencies outside this range, for practical purposes claims outside this range can be ignored.
As in all things audio, you need to listen to the gear to see if you like it - specs can't tell you much of the story.