Understanding which modes are at work in your room can be useful for an engineer.... for instance, if you know you have a 10dB peak at, say, 65Hz, you'll know to be very careful with that frequency.
However, at the end of the day the answer is always the same for small rooms: you need to add more broadband bass traps, which will flatten your frequency response across all frequencies. It's all about coverage area, the more traps you add to the more room corners (as a starting point), the flatter your room will get.
If you want to run a test to see which parts of your room should get bass traps first, then this simple test is very useful:
RealTraps - Filtered Noise