You're welcome; 'glad that I could be of assistance.

The whole 'trick' in determining the unknown value of the test signal was in comparing a file having a known dBFS value to the test signals produced by the 990 (for a given gain setting on the 990's volume control).

Having the (FFT) spectrum analyzer available simplified things because it made it possible to average each signal in the same way. That is, were one to try this using only a true RMS DMM (in order to compare the RMS values of the two files), the answer obtained for the value of the test signals would be varying (on the DMM display) due to the differeing crest factor as a function of time; this would make estimating its value somewhat of a crapshoot - you would be in the ballpark, but it would still only be a 'guesstimate'. For the signal that I created (a sine function), this isn't really an issue as such a signal has uniform crest factor as a function of time, so the DMM's display would always show the same RMS voltage.