Tvad's suggestion for the Audio Control C-101 Equalizer is good if you can get it for anything like $30. Don't forget that you will need to add a calibration microphone to fully utilize the C-101. I'm not sure what kind of mic is needed, but I have used an Audio Control "Richter Scale" (a low frequency range 1/3 octave equalizer), and its mic was unique.
Audio control equipment is well designed, and constructed. My Richter Scale served well for more than a decade, and, for that matter, still works. With an old unit like this with sliders (linear pots) you may expect that some of the sliders are noisy. This is usually correctable by a dose of contact cleaner from Radio Shack.
Using an equalizer is not as easy as you might suspect. You can go crazy pushing the sliders up and down! The Behringer DEQ2496 which I suggested will cost about $350 including microphone, but it is a far more capable instrument than the C-100, and one feature is an automatic equalization process that will keep you out of deep trouble. The DEQ2496 has balanced inputs and outputs, and therefore to use it with unbalanced interconnects which you may have (which is perfectly OK) you would need four XLR to RCA adapter plugs, or better yet, buy new interconnects with XLR on one end and RCA on the other, and wired unbalanced.
Others have pointed to deficiencies of your equipment and if they are right the equalizer will be counteracting these deficiencies. However, even if you replaced all the equipment your room's acoustic deficiencies would remain, and an equalizer will remain useful to correct these.