How do you know what room treatments to use?

Mike, I think most of the companies that sell room treatments will tell you that accurate diagnostics of acoustical problems is highly dependent upon measurement.  In the absence of measurement, about all they can do is sell you a treatment package and make suggestions about placement.  Sometimes that is enough to fix the issue.  How lucky do you feel?  Downloading the free REW software and learning to use it, if you haven't already done so, will greatly facilitate getting your room problems mitigated.  There is a learning curve, but not one that is insurmountable. 

Each situation is somewhat different.  Do you perceive the problems to be more high frequency issues or low frequency?  High frequency problems are usually not hard to fix.  Slap echo is common in rooms that have uncarpeted floors and plaster or drywall ceilings, and the lower the ceiling, the worse the problem.   Adding area rugs can be an attractive solution or at least partial mitigation for that problem.  Also, parallel side walls that are untreated can be offenders.  There are companies including GIK that offer artwork backed by fiberglass absorption.   Again, this can be an attractive solution for slap echo. 

Slap echo and/or a frequency response that is elevated in the high frequencies can make you want to turn the music down in a hurry.   In my own room, I'm still working on the high frequency problems.   Things sound pretty decent until average dB levels get up above 83 or so, then I start wanting to turn things down for a more relaxed and natural sound.   My room isn't carpeted, I have 8' drywall ceilings, and I have relatively little absorbent furniture.  I also have a lot of low frequency absorption intended to mitigate some pretty serious low frequency ringing.  By the time I got the low frequencies straightened out,  my overall frequency response was tilted up in the high frequencies. 

Another important question-- How much latitude do you have in placement of speakers and main listening position?  Those are the most important levers you have for dealing with low frequency problems.  You can do far more with placement modifications than you will ever accomplish with treatment.  Low frequency issues are hard to treat.  This is especially true if you are not in a dedicated audio room.  Low frequency absorbers aren't, to my knowledge, available in designs that look at home in a typical living room. 

I chose to go with DSP in my living room in order to ensure domestic tranquility.  But personally, I wouldn't want to introduce DSP into a high end system. 


  

Ditto the GIK suggestion.

Go to Youtube, search Acoustic Fields and start learning.

@ FALCONQUEST, +1

mglik, you can not tell without measuring. Some of us from experience can get a decent idea by listening but still measuring is the best. Now, since you are planning on getting a new preamp you might look at three units. DEQX, Anthem STR and Trinnov Amethyst. All three measure the environment one speaker at a time and will show you response curves. All three will calculate filters to make your frequency response flat then you can superimpose any equalization you like. All three have digital bass management so you can add subwoofers any time you want and integrate then in the best way possible. All three have four channels of very high quality DAC so you do not need an out board DAC. The Trinnov has a phono amp built in. You will still need some room treatment but much less and you won't be guessing. Nobody will be trying to sell you as much stuff as they can shove into your room and you will wind up with much better sound, imaging and bass. I personally like the Trinnov best but it is by far the most expensive. It is a toss up between the DEQX and the Anthem unit. 
If you are a plug and play guy and do not want to put any effort into this initially these three are not for you. If you love playing with your system these are a lot of fun and way more effective than any other tweak you can buy. You won't need any:) If you decide to go this way and need assistance feel free to ask.