Quite honestly, your measurements are not that out of the ordinary. Most speakers do not take into account room acoustics and are not designed for linear "in room" response. This problem is further compounded by improper placement or selecting speakers that are not really suitable for one's listening environment.
You have standing waves or nodes that are being excited with specific frequencies. These are resonances due to room dimensions and speaker placement. The resonances that are "in phase" at 32 hz ( bass reinforcement ) and out of phase ( causing cancellation ) at 64 Hz can somewhat be juggled and manipulated. Since these are harmonics of each other, you need to somehow stagger the nodes and break them up. Since you can't really just change room dimensions at the drop of a hat, the simplest solution is to move the speakers and / or your seated listening position.
While you can try all different locations for your speakers and not come up with anything any better, i would simply start by adjusting them so that they are NOT an equal distance from each side wall. While this will change the point of the first reflection from each speaker, that is more easily dealt with since those frequencies are more easily absorbed / diffused.
Once you can find a spot for each speaker that smooths out the response, you can further alter the response by moving the speakers closer or further away from the back wall. This can also be manipulated by changing the distance between your "sweet spot" and the speakers. If you doubt this, simply move the spl meter or your head forward a foot or two and listen / measure the difference.
By taking this approach, you will produce several smaller peaks and dips. Your measurements will probably look a lot more like a very wiggly line rather than the major hill and valley that you are currently experiencing. The end result should be something that is generally smoother in response and a little easier to deal with.
The use of good sized heavily padded cloth covered furniture that is strategically placed along with acoustic treatments geared towards low frequencies ( read this as large and very thick ) can also be put to work to your benefit.
Keep in mind that most "acoustic panels" are pretty worthless below appr 400 Hz. As such, don't go crazy thinking that you're going to solve your problem by buying some RPG Pro-Foam or something like that. All that type of stuff does is to create even more erratic response up higher in the audio spectrum. One look at the absorption co-efficient graph will tell you that they would not work too well for you ( nor in most other situations ).
If you really want to strive for a better "in room" response, try picking up a book by F. Alton Everest entitled "the master handbook of acoustics". It can walk you through why this is occuring, how to deal with, how to build your own acoustic treatments, etc... and help you to better understand how to achieve "killer sound" on a budget. Sean
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