The Empress Parametric EQ allows narrow, medium and broad band Q control which might help you locate and diminish the frequencies giving you a problem. this is reprinted from a sales web-site:
Empress Effects ParaEq w/ BoostThe ParaEq w/ Boost from Empress Effects is a super transparent parametric eq with true bypass that is perfect for adding a little to your guitar or bass tone. The Empress ParaEq features Hi/Mid/Low shaping complete with overlapping frequency selection, three different Q's to choose from, and up to 15db of boost or cut for each frequency. This gives the ParaEq the ability to do anything from mild sweetening, to room adjustments, to feedback control for acoustic guitars. It also features up to 30db of clean boost to push your amp to the limit or push a long effects chain. The Empress ParaEq also features an input pad enabling it to take any instrument or work in an effects loop.
Empress Effects ParaEq Features:
True Bypass
Hi/Mid/Low Frequency Controls
Hi/Mid/Low Gain Controls
Selectable Q for Each Frequency
Input Pad
30db Clean Boost
Selectable 9, 12, or 18 Volt Operation
Less than 0.03% Distortion
