Yes the 30 VDC is only used for the amplifier (LM379S).
All others voltages +/- 12vdc are for D/A converters (1408) and pre-amp. (LM741).
The A7 board (speech/sound power supply board) produce the regulated +/- 12 volts for converters, and the +30 volts for the amplifier.
The sound board had receive later some modifications:
On september, 1981, the "On-target newsletter" announce an improvement to provide higher volume level for sound/speech. They remove and replace some discrete components around the LM379S. Nothing very important here (they also improve the stability for the speech generator - but not installed on HH).
But, on july/aug. 1983, as the LM379S was no more produced, Gottlieb replace it by a TDA2002, mounted on a piggy-back. Of course, as the TDA2002 doesn't support the 30 VDC, they also made some modifications on the A7 power board. Only the "high-voltage" section was changed, to reduce the 30 VDC to 18 VDC to be compliant with the TDA2002 specifications.
Finding a LM379S today is very difficult. When the amp is dead, we use solutions based on LM1875 or something similar.