Cost to treat a room?

http://www.readyacoustics.com/

$815.95 maybe?

Also another question I have is, if I treat the room, then change my amp/pre, would I have to make any adjustments with the room treatment? All audiophile amps/pres are supposed to give a flat response, correct? So my guess is I would not need to change anything with the room treatment or remeasure unless I change the speakers. Is this true?

Yes it is true. BTW Room treatments do not really change frequency response of audio equipment so much as room treatments will reduce or remove the audible effects of reflections off the walls. That can make it sound better or not depending what the reflections add to the sound that you find objectionable or pleasurable..

If you treat your room properly you should be able to tune around system changes by adjusting speaker placement and toe in. If that doesn't do it then removing or adding treatment can be done.

After treating your room you are most likely going to want to play with speaker placement anyway as most likely you are compensating for the sound of the room with your current placement and toe in.

You can fake room treatment with a few pillows just to see if you want to put the effort in. I myself wasted a bunch of money in years past on 'warm' cables when what I really needed was treatment behind my speakers and at the mirror points and a touch less toe in. Ended up getting rid of the brightness and huge improvements in detail, soundstaging, and pace.

I would read up a bit before you do anything.
The Room treatment vendors sites that have been recommended have a lot of info. I also like Jim Smith's book "Get Better Sound".

I plan on getting the help of a professional acoustic engineer to help with my room treatment. Right now I have a pretty live room, with definite flutter echo. What can I expect post-treatment? Since the room will be "less live" than before, am I going to have to use more amplifier power to reach the same volume level? (I like to listen pretty loud, usually 85db) If yes, how much more amp power are we talking here? I'm using B&W 805S in a 15x17 room right now, and they fill the room with sound quite well. I am hoping they still can fill the room nicely after the treatment.