An interesting discussion, all the way around.
It's interesting to me that these boxes came AFTER the Dorrough DAP 310, didn't they? So a broadband compressor was released (and was popular!) for radio after multiband compression was already available... that's bizarre to me.
The Volumax/Audimax series came in more than one model, but I believe that I saw the final generation of them in late '74 and they were not new by then. The DAPs came along in about 1973 when the CBS Labs designs were starting to really show their age.
The earliest transistorized CBS Labs stuff went back to about 1965 so they predate the DAPs by almost a decade. In fact, CBS Labs had introduced an all tube version in the late 50s or early 60s but it was too expensive for most stations. The 400 series stuff was a direct, low cost, descendant of the tube version. In 1979, we still had the 400 series Audimax & Volumax on an AM station. One of those two (I think the Volumax) used one of those Nuvistors (actually, a miniature, metal cased, vacuum tube with a 67V plate voltage, the last attempt to compete with transistors). That 400 series had been on the air since the late 1960s and was finally replaced in 1982. On the other hand, the DAP310 was still hotly desired for AM service in 1978, years after the basic Audimax/Volumax setup was abandoned by those that could afford anything better. The DAP310 remained popular for AM until CRL and Orban cut into the market in the mid-80s.