Hello!  I need some help with some poor room-acoustics.  Basically, the more I know, the worse off I am!  So after noticing that mixing tracks has become increasingly difficult, I decided to perform some tests of my room studio.  I am experiencing some BIG peaks and valleys from 1kHz down to about 60Hz.  The lower bass frequency peaks are super-terrible, causing bad mixes altogether.  See below.  Because of this crap, I perfer to mix on--gasp!--headphones (with help from Waves NX)!
 

 
You may need to save each image so it can be zoomed in.  All plots are of a sine sweep 20kHz to 20Hz, Audio Technica AT2020 mic at the "head position," which is at the point of the equilateral triangle.  Note the big boomy peaks around 500-80Hz.  The same plot, but zoomed in, is below.  Between the largest peak and the adjacent valley at 120-100Hz is about 13dB!  In fact, a single-sine-wave test at 120Hz is VERY LOUD!  HORRIBLE!
 

 
I took one of the monitors outside in the yard and performed the same sine-sweep test with the mic about 3 feet away from the speaker.  This test was to eliminate any room-acoustical and standing wave issues.  The recording hardware was changed, however; now a Tascam DR-07mkII field recorder.  Though not the same quality as the studio mic, it did show a flat response.  See below.
 

 
Brought the monitor back inside, and performing the same test now with the Tascam recorder, here is what it looks like.  See below. Compare the plot below to the first plot and note that it is quite similar to the AT2020 mic test.  High frequencies are not as linear, but it did pick up the boomy peaks at the bottom end.
 

 
Monitors are KRK Rokit 6's on 36" monitor stands filled with sand.  The monitors-to-head triangle is very near 48" for all sides.  Monitors are close to the wall, about 1" from the back of the cabinet to the wall.  Desk and speakers are NOT in the center of the room (i.e. the R monitor is 2' from the wall and the L monitor is 6' from the opposite wall).  Room is 14'-3" x 16'-6" but not completely square.  See below.  Room is partially treated for high frequencies with custom-made 2'x4' Auralex 2" foam panels on either side of the desk and behind on the back wall.  No bass traps at this time.  Wall-to-wall carpet on floor.  Furniture and other stuff in the room.
 

Given the side of the desk, and windows on the left side of the desk and along the long wall, with wood shutters on them that open inward, the desk position in the room is limited.
 
Looking for some guidance to what I should do to get a better sounding room, and thereby creating setup and sound I can trust when mixing.  Because of the fairly low frequencies causing the problems, I don't want to invest in bass traps and/or make them myself with Roxul or "703" panels only to find out that it was completely in vain!  After all, the corner traps, if truly needed, will most likely need to be very thick.  Thanks!

I think the acoustic foam retailers will give you a detailed diagram of what kind of treatment you need based on your room dimensions. 

couple quick wins - reach out to GIK Acoustics for some help in getting the right amount of absorption into your space. also, center the desk. positioned as you are, the right side will experience additional low freq "compression/boost" in that corner. treat the corners with deep absorption. 

When examining those plots, keep in mind that it's not just the amplitude of the peaks that's important, but also their width and duration. The narrower the peak (or dip), the larger it has to be before you can hear it (and the harder it is to mitigate). Also , the longer a peak rings out in time, the more problematic it is - but we can't determine that from the measurements you made; you need a waterfall graph to see that.
 
To see what I'm talking about with amplitude vs. bandwidth, try to find find a reliable (meaning not from the manufacturer) frequency-response graph for your headphones. Chances are it has worse peaks and dips than your room! But if they're quality headphones, those anomalies probably aren't a big problem because their widths are narrow.
 
The graph you posted doesn't really look that bad at all. That 150 Hz peak looks huge mainly because your speakers drop off so quickly below it. I wouldn't expect any more from 6" speakers, and I wouldn't add a sub in a room that size to extend bass response because it would cause even more problems. The reason you prefer headphones is they have deeper (and smoother) bass extension and are not subject to ringing.
 
As you are aware, the foam or fiberglass treatments you currently have in place don't do anything for low-frequency problems. Unfortunately, absorbers that would be effective at 100-150 Hz would also be too bulky for your room (they'd have to be 2.5' thick in each corner). The alternative would be tuned resonators, but now you're into need-a-calculator land. And need-a-credit-card land.
 
The only practical action is to somehow get your speakers further away from the wall, even if it means wasting some precious space. I built a vocal booth between my desk and the wall. The space isn't wasted, and my speakers are 6' from the back wall with lots of fiberglass in between.
 
You might also consider some absorption behind you if that's practical. Bear in mind that those peaks are partly caused by nonlinear reverberation. Lower the overall reverb time in the room and that will smooth out your variances. 
 
 

actually, if you place a 4" thick trap diagonally across a corner - leaving air space behind it, or filling it with light insulation, you'll find the absorption can be effective even as low as 70-80hz. you do want the corners treated floor to ceiling though for the most effect. but you can also start off small - say 8 4'x2'x4" absorbers - 1 in each corner, 2 on either side of the desk, 2 overhead as clouds. if you cannot put one in the corner by the door, put one on the door itself. if you can space it off the door by 4" or so, that will help as well.
 

Suspending heavy rockwool panels a few inches below the ceiling can make pretty good bass (and treble) traps. As can rockwool put across the corners where the walls and ceiling meet. Panels can also be spaced off side-walls and doors with plasterboard lathing (UK) or 2x4 (US). Edging them with timber can help protect the edges. If you want them to look nice cover them with hessian.

Acoustic foam tends to be expensive, foam that's any good tends to be very expensive and heavy. Rockwool is (comparatively) cheap. Don't overdo things either, a room that's too acoustically dead is horrible and weird to work in and not necessarily a good recording room either.

Before doing anything, try shifting your listening position and see if it makes a difference. Same with the monitor positions. Sometimes moving your head or the monitors as little as a foot or even a few inches can make quite a difference.

It can also be worth using an inexpensive decibel meter with equal volume sine waves at a range of frequencies to plot the room curves as well. The mics in them would be dreadful for recording, but usually work well for the job they're designed to do. And no studio, however small, should be without a dB meter, they're handy for all sorts of things like setting a consistent monitoring volume.

Or you could just see what ARC2 or Sonarworks does for you. 

<curmudgeon mode ON>
 
I've yet to see a demonstration of any software room correction tool that can correct problems in a small, square room. So my advice would be to hold off on investments in tools like ARC and Sonarworks... there are some laws of physics that software still can't violate<G>!

Placement and Room treatments - absorption, diffusion, and reflection - will only get you so far. A room with equal room dimensions has physical problems that can not be fixed in software nor with treatments. HOWEVER, the problems can be minimized, and quite often reduced sufficiently that you can do good work. And ignoring placement, absorption, diffusion, and reflection is a pretty good recipe for frustration<G>!

Which overlooks the fact that some of our favorite records were tracked and mixed in control rooms that make the average bedroom studio look downright brilliant. But that's no excuse to leave a room alone, I mention it for the sake of completeness.

You CAN learn everything you need to know about small space acoustics from a variety of textbooks. You may be able to learn some of it on the web, but I wouldn't want to spend the effort sifting good from bad.
 
Placement - placement is the king/queen of tools, get the placement as good as possible and you'll need less treatment (most of the time, I am sure there are rooms that prove this by exception<G>!)
 
Absorption - keep it as wideband as possible - tuned traps introduce as many problems as they solve. And don't (as others have said) over do it. A dead room is often more difficult to work in than a live room with problems.
 
Diffusion - Diffusion requires distance to take effect. Different patterns require different distances. Find out if you really have the space to use diffusion. And remember, in addition to all the really cool math based diffusors there are book shelves, poly-cylindrical diffusors, and other tools at your disposal.
 
Reflection - Reflection does not get the respect it deserves! Sometimes it really is as simple as redirecting the energy to someplace where it does less harm.

If you are going to make a living as a composer or producer I still think an experienced acoustician / studio designer is the way to go. Curiosity is a bugger though, and I'm glad I had to learn about small room acoustics. It is fascinating stuff.
 
Same goes for the treatments - if you are making a living from the room invest in products from reputable manufacturers. If you are not quite there yet then have fun making them yourself.
 
<end curmudgeon mode>

Thanks for all of the replies and information, fellas.  Sorry it took me this long to reply, but I've been out of town and typing/posting on the phone is a PITA!
 
I've redeveloped my room's and desk's measurements in AutoCAD much better, and I intend to see if I can move things around a bit to see what happens.  From what I've sifted through online, the recommendation is to have the desk face a "long wall."  Given a big window area with opening plantation shutters, the desk is sort of stuck in the corner, as seen in the pic in the first post.  I can rotate the desk and monitors 90 degrees so that it is against the "long wall," but then the back of the left monitor will face directly into a corner; probably highly undesirable!  It will, however, be almost 2' from the wall/corner, so perhaps that will help as well.  The keep the orientation as is and pull everything back from the wall another 2' or so.  This introduces additional problems as my 30" monitor is mounted to the wall via a radial-arm-swivel-mount, which in turn sets it back off the desk's edge so the viewing distance is more appropriate (i.e. 36").  If sweep tests confirm an improvement is made, then I can always put the monitor back on its stand and set it on a small table.  Wait, maybe that old keyboard stand with an added shelf can be used!

Forum is messed up.  Posts not working properly.
 
Working now.