In cinemas the test signal is wideband noise at -20 dBFS, set to play at 85 dB spl. That was much too loud and shocking a noise signal for home users, so we (Tony Grimani and yours truly) decided to define a -30 dBFS narrow band noise signal. The end result is the same: when the volume control is set for 0, it is playing at reference level (ignoring Dialnorm...).
As for the SSP's internal noise, back when I bought my SSP-800 in late 2009, I noticed the internal noise was apparently intending 85 dB. I brought this to Classe's attention and it was corrected in a FW update.
The test signal is generated at -30 dBFS, so you should set your speakers to 75 dB if it is your intention to calibrate the volume control per THX. This is easy to confirm. After calibrating to 75, play a THX test signal, and set the volume to hit 75 dB spl. Look at the MV setting. It will be 0 (or darned close). I have confirmed this on numerous occasions.
Of course this calibration to a known spl is not required at all. It has no effect on sound quality (assuming all the speakers are balanced properly), only on what the volume control reads out.