you run before and after. REW is just measuring the response (and can also suggest filters). It is really about validation of what filters or RC systems are supposedly doing.
Pre-Pro: Emotiva XMC-1, DSP: Mini DSP 2x4 balanced
Amplifier: Sunfire TGA7400, Lab Gruppen FP10000Q
Speakers: JTR Noesis 212HT (LCR), JTR Slant 8's (surrounds), 3 Orbit Shifter LFU's
Sources: PS3, HTPC, Dish Network
Projector: Epson 8350
Screen: Seymour XD AT 138" diagonal 16:9