I think he's referring to how different songs are recorded with different acoustical properties, or how stations broadcast. For instance in my area, 101.9fm broadcasts a lot of bass and mild treble, so I have to adjust accordingly. Whereas 104.9fm plays the exact opposite and I have to adjust opposite. Then 105.7fm plays just fine and requires no adjustment.
His question is, do the headunits automatically adjust for this?
I think the answer is "No, they suck just enough that it levels itself out. Installing a new HU brings out those imperfections that the stock one got rid of due to its suckiness."