kidcarbon, your room seems big, lucky you. Analysis of a room's behaviour is ultimately done by the brain, not a single tool. As Andre has shown there are other things to consider. So let's take them one by one-
Big room, primary modes are below music, modal density should be decent.
Any problems will be due to similar dimensions. 24 and 20 are both divisible by four. What is your height to the hard surface above your ceiling tiles?
Lets see the predicted modes in a calculator. http://www.bobgolds.com/Mode/RoomModes.htm
Next try playing Sine waves at the trouble frequencies. REW has a nice smooth frequency sweep for this. Generator/Frequency Follows Cursor.
Carefully tuning to predicted modes, (the frequencies will be slightly different) use an SLM to measure peaks and nulls in your listening area.
Make a map. You now know how where the modal trouble spots are and how strong their effect is. This give great advice on where to sit or place speakers and of course where to place traps. You will get great modal readings in the corners, in itself a very tactile confirmation of the theory.
Reflections from all boundaries near the speakers can cause nulls and peaks.
Andre has pointed out one, the floor is another possibility.
Take a look at Thomas Barefoot's Wall Bounce Calculator. I haven't worked out how to direct link but you can find it in Acoustic Resources on my site.
Observe how the frequency dips caused by the side wall, ceiling, or floor, reflections changes as you move the listener position, but the front wall one does not. That will get your geometric juices flowing.
Frequency Response- Looks like your speakers are pretty small. Try them much closer to the front wall. The bumps may get worse or not but some low bass is better than none IMHO.
The modes in the Waterfalls are sort of cut off. Furthermore some of them seem to be not changing in level for some time before that.
Maybe try some sort of zoom or threshold adjustment, level or time, to see them more clearly. I like to see them vanish into the noise floor, which can be quite high if you have computers and such in the room.
Note small changes of position, say between the left and right ear can yield quite different results. You need to take a broad view of this, ideally use spatial and even time domain averaging.
ETC- The geometry can fool us, or me in any case. It is good to confirm which reflection your are observing. The Mirror Trick is useful. The trusty measuring tape (or laser pointer) of course, but add a piece of string.
Perhaps the most definite way to confirm a reflection is to block it or place absorption in it's path.
I use a panel of 703 wrapped in fabric for this. No frame, light.
I think REW has either a Real Time or Repeat function which should make this very easy.
Lupo has also given us an ingenious method of confirming reflections.
The thread is called Useful ETC Tricks or such I will return and edit it in here.
Perhaps he will spot this and may have other ETC info. When I have ETC questions, I ask Lupo.
Your Room seems big. You might consider changing dimensions, i.e. a wall if the height is related to the others, e.g. 10 feet would be ugly.
Or you could consider Gobos or other portable Traps simply assembled around your mix area. A suspended ceiling is a great asset. You could install enhanced absorptive tiles (705) and fill some of the void above with light fluffy insulation.
This will hit floor to ceiling problems, modal and flutter. It will also somewhat relieve the pressure of the other modes by allowing them to expand and be absorbed vertically.