Every room is an equalizer, adding its own sound to the recording you're listening to. Suppose you find a narrow peak at 46Hz that is resulting in one-noted bass. How will diffusers and a SPL meter help you pull down that peak and get bass that is more even? Dubbing stages are tuned to industry standard curves using sophisticated measurement gear and equalization. Earlier you said "using my Radio Shack SPL meter I see no reason to step up to the 998 just for room correction", which seems to conflate calibration with room correction. They're not the same. Using a SPL meter to set speaker levels is different than sampling the sound at multiple locations and equalizing away the room's negative effects.