I think maybe the reason for the decline you're seeing is due to these online discussion groups. The date range you're talking about coincides with the introduction and widespread growth of BBS and HTTP on the internet.Personally, I love rot rod car and audio magazines. I loved computer magazines too back in the 70's and 80's when they were into the nuts and bolts, schematics, source code and assembly language. It's great to be able to thumb through the pages, take it with you wherever you go, and keep it for ever and ever when you're done.
But now days, lots of people are on the internet and it has it's own advantage, being interactive. I was amazed to see how much activity there was on audio and automotive discussion groups when I first found them. It has definitely had an impact on printed media.
So there are cool things being published in both media. But the point I wanted to make here is about online media. There's a ton of stuff happening here, much more than a half dozen speaker projects a month. If you look around online, you'll probably find that many per hour. And I'm not talking about just cheezy thrown-together stuff either, I'm talking about serious, well-thought-out efforts.