Input capacitance adjustment

I have an Audio Research Ph3SE and a manual that is ever so cryptic in how to adjust input capacitance (currently 90 picofarads). Input impedance is easy and they provide the resistors. Capacitance is more of a mystery including where to get the capacitors. Any advice is appreciated.
Dear Dorrman8: You always can do a phone call directly to AR: Audio Research Corporation
3900 Annapolis Lane North
Plymouth, Minnesota USA 55447-5447
Phone: 763-577-9700 Fax: 763-577-0323

regards and enjoy the music.