Trick or treat?

[rolyasm]

Hello everyone,
Just got finished using and abusing this site to help me finish my sub. I ran my first tests with REW and did my first EQ with the BFD, PDQ. hehe. Anyway, my question is: Now what? Do I start to add room treatments based on my response? I have another couch to buy and some curtains to put over the windows and I am sure this will effect my response. So... do I just wait, then run some more tests and see what it looks like? How does one determine when treatments are needed? Say I had a poor reponse (which I don't know how that looks), how do I know what kind of treatment is needed and where to put it. Is it a trial and error thing, or can you look at a response curve, look at the room, and say, "hey, I need a bass trap here?"
Just for kicks, here is my first EQ's response and the response from listening position with no EQ.
Roly

[rolyasm]

Also, my dedicated HT room is 18 wide 24 deep 9 high. On the left side of the room are two windows. I know, a dedicated room with windows. Couldn't be helped. Anyway, I was thinking of covering the windows with something permanant, like a bass trap,then covering the trap with curtains. That way the windows are very well covered and the room dark. Could applying traps in random positions like this cause things to sound worse, and if so, could I use something like cardboard as a nice seal for the windows?
Roly