> What was the API 325 line amp actually used for??
Line amps drive lines. They make almost a watt or more, may or may not have tons of gain, may nor have lowest-noise inputs. They pump signal out of the box, out of the studio, maybe across the state.
The partner is an input amp. LOWest noise, good but not excessive gain, maybe not quite 1/10th watt output. Just brings weak signals into the box, where you fiddle it and pass it to a Line Amp to a destination. Also used to boost signals that have been fiddled too much (lossy mixing networks, passive EQs, etc.)
In the 1920s, you would have intermediate amps as well.
By the 1940s, they consolidated on Input Amps with low noise and modest output, and Line Amps with ample output to drive several lines and pads at high level.
Around 1960 you find the two functions merging: one amp module with low noise, ample gain, and ample output power.
Early transistor amps could just about get low noise OR good output, but didn't get both in the same module. Later amps did, and the difference between an Input and a Line amp might just be whether the mike transformer was included.
I suspect the beast you refer to can be either/or, depending on transformers. Older transistor amps need input trasformers for lowest noise from 200 ohm mikes. They don't need output transformers to make power in 600 ohm loads (as tubes do) but any line not dedicated to another input may want a transformer to break ground and balance the signal. In small systems you might have 6 input irons and one output iron; mega-systems might have iron everywhere.