Adjust resistance & capacitance to

Hopefully, pretty straightforward. Vinyl sounds GREAT with one amp I have. With another that has a more extended high end, I notice some harshness on certain LPs. Was wondering if anyone had experience adjusting capacitance and/or resistance to smooth overly bright treble. My phono-pre has switches to accommodate different cartridges - both MM & MC. If yes, any advice on the direction to move things? Thanks in advance.