Yeah, usually I associate "vintage" with a very mid-heavy sound, actually.
I don't know where this idea that bassists back in the day only had lows going on got started, but it's pretty inaccurate.
Sure, like you mention, there weren't really any tweeters in bass rigs at the time, but that doesn't mean there weren't any mids and highs at all.
Preach it!
I swear it must come from the same place as those daily "How do I get my brand x bass to sound like a Precision" threads that inevitably suggest to make any bass sound like a Precision all you have to do is suck the life out of your tone and VIOLA!

I guess the common and persistent perception is that all bass was back in the old days was a generic thud.
Anyway, good point about the mids.