I think it is because when i hear amplifier i think of speakers.
In the not so distant past big distributers had catalogues you could flick through
TI still has a great book - The Logic Pocket Data Book http://www.ti.com/lit/ug/scyd013b/scyd013b.pdfWhen I was a young engineer (in the mid 80's) we used to have...
When I was a young engineer (in the mid 80's) we used to have bookshelves full of things like this from TI, National Semiconductor, Intel, IDT, Motorola, AMD, Atmel, etc.
maybe I'll get to it somehow and post a photo. Gobs of app. manuals, etc., from RCA, Sylvania, Fairchild (I think), on and on. Plus tube guides, HAM, RC, ... too much to list really.