I use a soundcraftsman octave equalizer in my system. It is 80's analog, but it gives me the ability to adjust, albeit imperfectly, for a given recording/signal source. Cassettes tend to be a little weak in the high-end, while some vinyl is overly heavy in the low-end. Having an equalizer allows one to smooth out the frequency that is heavy or light. The best way to think of it is it makes you the recording engineer and allows you to adjust the material playback.
How you adjust it is entirely up to you and your ears.
How you adjust it is entirely up to you and your ears.