Originally Posted by anandogs
Ok thanks it sounds very useful. I am, however, just a beginner: Would you still recommend it?
Ah the eternal question. Well, EQs (and all equipment in general) can vastly overcomplicate things if you don't take the time to figure out how they really work to affect your playing.
In an effort to simplify things as much as possible without chucking a lot of cash, I think the simplest fix - and the one that will require the least amount of knob twiddling - is to angle your amp in a different position within the room in an effort to affect the standing wave reflections you are hearing.
Another simple suggestion would be to try dialing back your midrange knob in an attempt to pull out the frequency that is causing the problem.