My first "bass amp" was a Scott stereo receiver that looked something like this one:
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It went through a couple of Advent stereo speakers. I recall plugging into the headphone jack, but I'm not sure how that would work. Apparently it did, and nothing blew up. The receiver got stolen from my apartment in NYC in a break in in about 1983. It had survived all kinds of abuse.
My first "real" bass amp was one of these:
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I got it for $100 and the free-bass was thrown in because the guy I bought the amp from wanted to get rid of it after he found it in the bathtub in an abandoned house. The amp sucked, but at least it was more or less intended as a bass amp. I sold it many years later for $100 to a guy who knew exactly what it was and wanted it for guitar. Go figure.