The pre-AGC is an extra AGC with a different threshold. It was made for voice tracks that are for example at +10dB in the mix. The pre-AGC only acts on the very loud parts allowing you to use a fast moving AGC on voice tracks and a slower AGC on music.
Low level boost is intended for classical music with very quiet parts.
For pop/rock/electronic etc genres you probably don't need either.
Thank you for clarifying! However, my use for ST is to process all computer audio, including very poorly recorded youtube videos and podcasts. So the Low Level Boost seems to help bring up the levels to the point where the AGC can take over. I realize it's intended for classical music, but as usual, I'm trying to use ST for purposes for which it is not explicitly intended.
Does this mean that the improvements to the AGC (progressive ratio and dynamic speeds) would not be effective for the Low Level Boost's intended purpose? Perhaps I should keep tweaking the AGC or do some pre-amplification.