Modifying your amp by yourself without having a schematic of something that have been done to an exactly similar amp before is pretty comlpicated and can lead to undesired results ... Don't even think about it unless you know what are you doing.

EQ's are a very productive tool , you won't believe how can they dramatically alter your tone unless you try one .
Some people even use more than one EQ pedal (before the distortion , after it ,etc.) ... If you're thinking of buying one , I believe the Boss pedal is a good choice.