The advice to acquire a mic and download REW or HolmImpulse, as used by Earl Geddes and also free, is solid. Playing with this allows you to see the results of added treatment.
The basic problem all rooms have is the sound taking too long to decay and the use of EQ simply can not reduce the decay time, it needs to be absorbed and the OC703 you bought is great for the job.
tomic601 mentioned RT60. This is the time it takes for the sound bouncing chaotically around your room to decay by 60dB. and without looking up the figure for your size room would, I guess, be about 400ms. So if you see some frequencies with a T60 of 1 sec. these are the ones to be tamed. Then after introducing and/or moving some more treatment take another shot until you achieve an even smooth response across the full spectrum. Great fun.
By doing it this way you avoid any guesswork and will come to realise that the suggestions to hang a drape or install a thicker carpet is like trying to win the jackpot.
The thicker the absorber panel the greater the range of absorption. So 6" will be more broad-band than 4" Move away from the back wall and use absorbers. You would be too close to a diffuser on the back wall.
Have a good read of this:
https://ethanwiner.com/acoustics.html#bass%20traps