Looking at the owner’s manual for the 7075, 7125 and 7200, I find that the input voltage needed to obtain the full output voltage is 1 volt for the 7075/7125 and 1.43 volts for the 7200. So if you’ve got a volt-meter that will read AC sine waves and a test disc (or PC running (free) software) to generate a sine wave* somewhere between 40Hz and 120Hz, turn on your 970 and turn off your 7075, then measure the sine wave voltage of the center and/or mains outputs from the 970. If the voltage is nominally 1 volt, then you know that if your 7075 were turned on, you’d be getting nominal full output. The same applies to the 7125. If you upgraded to the 7200, you would need to be able to read a nominal 1.4-volts on the output(s) of the 970 in order to obtain full output from the 7200.

* Please note that the average volt meter, analog or digital, does not measure complex waveforms well - their highest accuracy for AC is as a sine wave near the usual power line frequencies - and that they usually show the RMS value, not the peak value. For pure sine waves, multiply the RMS voltage by the square root of 2 to find out the peak voltage. If your calculator doesn’t have square root, use 1.4 for a close approximation. If the meter shows 0.71 volts RMS, the peak voltage is about 1 volt.