Cost to treat a room?

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?

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.