How do you know what room treatments to use?

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. 

Talk to pros.  Call or e-mail GIK Acoustics. They have a section where you can upload pictures and ask for professional advice.  No one I know has better products and advice for less.

You could do them yourselves but wrapping fabric is no fun.  I have GIK panels.  I have 11 and cost about $1500 inc.  freight.  The most bang for the buck are the walls between you and the speakers known as first reflection point.  If your system is in a dedicated room, it is easy.

My converted bedroom (10x9x8) sounded like cr*p. It was just awful. I called GIK and sent them pictures of my room. They made several suggestions which I incorporated into the room. It was transformative. My room, despite its dimensions, now sounds very, very good thanks to GIK. I recommend them highly, not only for their good advice, but for their excellent quality and affordable products.

My room treatment with passive materials is homemade and not ideal but work well... I use only my ears to create it...(total cost under 50 bucks)

But my active device controls of the room acoustics, is totally astounding and cost little more but anyway peanuts....

In acoustic there is 2 complementary ways: the room treatment with passive materials, and the active controls with resonators and schumann generators grid all connected....(S.G. for 10 bucks and i own 10 in my grid). You can use a computer to calculate the best nodes in the room where to put the materials, but you must use your ears to synchronise the active controls device grid.... It is easy tough....

Except for some totally acoustically engineered room (very costly), active device controls of the room is the way to go for most of us....

Nobody will believe that but i say it anyway and if you want to know read my thread....

No one it seems know about this and nobody use these active device controls that free the sound from the room obstructive topology or furnitures and erase the speakers from the sound projected 3-d presence.....

My last control device cost me 15 dollars and it is the more powerful....If i say what it is, some will call me nuts.... Then if someone is curious it is in my last posts..

All aspect of sound are modified by active controls : dynamic, imaging, soundstage tonal accuracy, higher frequencies extension, bass extension etc....

Ok my duty is done....

A last word, the acoustical embedding of an audio system is more powerful for S. Q. than most upgrades, speakers included, and it is the more powerful of the 3 embeddings controls....

Nobody can listen to his speakers, we listen the room, because our ears are in the room not in the speakers enclosure and the sound waves we listen to is a complex integration of the direct waves and the reflected one .... The active device control work on this complex wave by resonance modification and amplification of the direct wave and synchronization between the direct and reflected one... This is my explanation and not science.... I know little but i am creative and i know how to listen....Anyway my results are amazing and cost peanuts....

Someone not knowing that about acoustic dont know really his own audio system ....

i know mine.....

:)