Sound correction in small room

You are correct in your understanding of what "room nodes" are.  I do not understand the math behind room nodes either, but the amroc page was really intended to show you what the "node frequencies" are and where they are the loudest (most pressure).  The locations where they are the loudest are the best places to put bass absorption panels (which are generally located in the front/rear corners).

If you were buying GIK, I recommended the Monster Bass Traps with the FlexRange Limiter because it excels in absorbing the 80-120 frequency range and it also does not absorb as much mids/highs.  You could get the triangle shaped bass traps if you wanted (these are the GIK TriTraps).  They will have only a little bit better absorption in the very low frequencies (such as 40-60hz), but it is not much better than the Monster Bass Traps.  However, the Monster Bass Traps with the FlexRange Limitor have a significantly better absorption in the 80-120hz area.  See measurement reports here:

http://gikacoustics.com/wp-content/uploads/2013/11/RAL-Report-FRT-Range-Limiter-Monster-Bass-Trap.pdf

http://gikacoustics.com/wp-content/uploads/2012/11/RAL-Tri-Trap-J-Mount.pdf

To give you a point of reference, I have 14 sound panels in my room.  Only 4 are full absorption types:

6 - Owens 703 FRK 2"

4 - Roxul Rockboard 80 2" (full absorption)

2 - Monster Bass Traps 7" with Flexrange Limiter

2 - Limp Mass panel bass panel tuned to 63 hz

Four of the 703 FRK are partially covered with towels - this was me tuning how much high-frequency reflections I actually had in the room.  The FRK panels actually worked very well in doing bass absorption down to 70-80 hz and would be an excellent alternative to the GIK Monster Bass Traps.

I did not double-up my FRK panels to make 4" panels.  I used black Guilford of Main fabric to cover them, which is more expensive at about $14 per yard from ATS Acoutics.  It requires 1.5 yards to cover one 2'x4' panel.  I just wrap the cloth around back and use spray glue to glue it on the back side of the panel (no side frame).  You could use duct tape if you wanted to.  The cloth is purely aesthetic and has no impact to the acoustics.  You can get very cheap burlap cloth (it would look crappy), but it would work fine.

Look in the ATS Acoustics clearance section for some deals.  They are currently selling grey burlap for $0.50/yard.

If you are under a tight budget, a 6-pack of 2" 703 FRK panels is $109 plus shipping from ATS Acoustics.  They are an excellent source and I have used them several times.  This will allow you to completely fill your front/rear corners (you can cut a panel in half to make a 2x2).  Just sit them in the corners at an angle.  They are VERY light and stiff enough that you don't need a frame.  You can use your existing 703 for 1st reflection or SBIR points.

I think the Monster Bass Traps with FlexRange limiter will probably be better, but they are significantly more expensive at $127 per panel.

Thinking about this further, the very low bass is always hard and expensive to treat.  The general broadband panels (even if they are very thick) are only average at absorbing the bass.  However, if you know the target frequency, you can get an effective panel.

We know that you have a room nodes at 55hz, 75h, 90hz, 110hz, and 111hz.  The FRK or Monster Bass Traps will effectively handle anything 75hz and above.  If you wanted to treat the 55hz problem, I would call GIK and have them make you a custom Scopus Tuned Membrane Bass Trap that is custom tuned to 55hz.  They have off-the-shelf models at T40, T70 and T100.  You would just be having them build a custom "T55" model (they will do this).  I suspect the price would be somewhere between $230 and $250 for a 2' x 2' panel.  It would probably be about 7-8" thick.  I would get a couple of them (at a minimum) and put them on the side wall floors near to the back corners (since amroc tells us that the 55Hz node is loudest on the side walls).

This is somewhat similar to my limp mass panel that is tuned to 63Hz.  I also plan to make another two limp mass panels that are tuned to 50Hz.  In my room, the 50hz also occurs on the side walls.  I will say that all my treatments have significantly increased the volume and definition of bass.  I am sitting at about the 10 foot spot in a room 17 feet long.

At 11:57 of this youtube review:

https://www.youtube.com/watch?v=nqgWHPv26L0

“A lot of people are going to look at this speaker and ignore the fact that the midrange is being handled by two 6” woofers and a tweeter array… and what that means is unless you have a big powerful amplifier, you’re not going to get the kind of dynamics out of this speaker that is just going to punch you in the chest.  Some people are going to be disappointed with that, some people won’t care, but you at least need to be aware of that”

 This is also going to be more constrained if you got the 4ohm version of the Double Impact, where your 20 watt tube amp may have a little more problems handling it (i.e. it’s not going to be the same as with a high current Class A/AB amp with a big power supply).

 He also talks about how a good amount of the midrange is actually reproduced by the outer ring of tweeters (6 tweeters).