Mark, I expect your negative dB readings show values that are that much lower than the dB level you calibrated. Did you calibrate the SPL level normally, with the REW test tones, first? And then disconnect the REW outputs from the receiver?... So, I am wondering...did I set something up wrong, and if not...what's negative dB? Also, I notice that the scale on the graphs bruce shows are in dbfs, but mine are in db...how does one get the data in dbfs vs. frequency in case I wanted to do that? ...
Under REW v4, the dB vs dB FS choice is a pulldown box selection at the bottom of the Spectrum window.