100% You should be using those mic filter screens. Remember, in a less then ideal room, the less level you fire off into the room the better as it will be less activated.
In a less than ideal situation like this you aim for bass trapping first. Prioritise 3 points corners, then 2 point. Fill each of then. Then after that you just make it dead dead dead and go for maximum absorption.
This is not the approach you would use normally but for a situation like your that is what will give you the best result.
Once this is done your remaining problem area will be 130-300 hz, which you will be able to address nicely with EQ, as your bass has been trapped and your HMF and highs have been contained.