Room modes, standing waves, resonances. That's what you've got. It is common for all rooms and room sizes.
Your room is very small and it shares common dimensions (height and width). This causes double/triple modal incidences. (This is where modal resonances of different dimensions are at the same frequency.)
Room volume = 810 cubic feet - This is about half the minimum recommended volume. Therefore, you will need a very large amount of trapping.
168.92 Hz *** Here's the one you really noticed.
219.63 Hz .... you get the idea.
These can only be eliminated by moving walls. or changing rooms. Due to these doubles you will need even more
Substantial trapping will smooth these out and lower their 'Q' (bandwidth) but will not
eliminate them. With proper trapping (That means a REALLY lot!), you can make this room workable, but it will entail considerable 'work' from you.