Edge22 - congrats on your GIK transaction and its impact on your room. You're well on your way to improving your room/system sound! I would HIGHLY recommend you buy and read Dr Floyd Toole's latest book (http://www.amazon.com/Sound-Reproduction-Acoustics-Psychoacoustics-Loudspeakers/dp/0240520092/ref=sr_1_1?ie=UTF8&s=books&qid=1269601512&sr=1-1) before you spend anymore money so that you (1)buy and/or make the right kinds of products, and (2)better understand where to place treatments and why they work the way they do.
A member above recommended the LEDE (live-end, dead-end) style of treating a room -- I'd stay away from that and here's why: The LEDE concept is something that came out of the 1970's from Don/Chip Davis/Syn Aud Con for recording control rooms. It's based on mostly ill-conceived ideas that stem from a misunderstanding of the Haas effect, which describes the level at which a delayed reflection sounds equally loud to the direct sound. The Hass effect has nothing to do with the audibility of a reflection which is 30-40 dB lower in level than the Hass effect. I would not recommend an LEDE room since they sound very strange, they do not achieve what they claim, and to work well you need to add tons of absorption.
Toole's philosophy with acoustical treatment is to diffuse or absorb reflections that do harm at the listening area, and leave alone the ones that do potential good. Everything else can be ignored since they never arrive at the listeners' ears or if they do, they are well below the threshold of harm.
Also mentioned by Schipo above - who's obviously read Toole's book - is that the lateral side wall reflections are beneficial, particularly in stereo, since they can produce greater apparent source width and spaciouness, while the front/rear reflections can decrease these attributes since they come from the same direction as the direct sound, increasing the Interaural Cross Correlation Coefficient (IACC). This is why the back wall is often treated first and also because you're sitting closer to it so the audible affects will he heard more than front wall treatment due to larger atttenuation caused by signal propogation loss and the repeated surface impacts on the sound's way to the front wall. If you want to have more focussed imaging you can always choose to absorb the first side-wall reflections, personal preferences prevail.
Moreover, diffusion and absorption should be wide-band to ensure that you are attenuating the entire spectrum of the reflection rather than simply applying a low pass filter to the reflection, which causes timbral colouration. So you need 3-4 inches of high density fibreglass to work down to 200 Hz and 2-4 ft wide-band diffusers.
Edge22, you may wish to explore the DIY route as an effective yet economical way of pursuing further treatments. For example, I've got 8 GIK Tri-Traps in a single back wall corner making a 7' high rectangle shaped bass trap that's 24" wide by 24" deep with an air space of 6-7" behind it which cost $$$. For the same money I could have built way more traps AND diffusion . . .
If you want to know how to build a Skyline or Hemi-Cylindrical diffuser - both of which are excellent at preventing a 'dead' sounding room - then email me.
Good luck!
