Capacitor

Why do you think changing caps will give you the desired results?  It's the entire preamp design, not one circuit component, that makes the difference.

You'd be better off selling the unit and searching for a different one that will give you the performance you want.  If you change one cap in the circuit, the manufacturer may decline to make repairs to such a modified unit.

Then why are you trying to correct an obvious design error rather than looking at a more carefully designed instrument?