It was called the Advent 500 Sound Space Delay I believe.I don't remember how many milliseconds it would give to the rear speakers for the hall effect,or the rest of its settings.
advent
back in the 70's-80's advent made an equalizer of some sort it came with 2 small speakers.was designed to be used as rear speakers in conjunction with main front speakers.the equalizer unit had 4 or 5 settings as i recall such as club,concert hall etc.wondering if any one has any info on this unit.