Back in the day, before the Internet, we used to do listening sessions among friends. Some were associated with audio clubs --not so easy to find unless you are near a large metro area--and some were entirely informal--just a hang at someone's house, spending time listening, chatting, maybe a meal. Different hosts had different systems and occasionally (more often with the formal audio clubs) equipment was brought in for evaluation. That's one way to scratch the itch.
I also like to do a lot of primary research. For example, if you are a member of the AES, you should be able to get access to their archive, which is extensive. There's lots of good info, particularly on vinyl LP manufacture, which is informative. There are also secondary sources that can lead you deeper into historical and antiquarian designs- e.g. Lansing Heritage. Some have user fora. Others, like Hi-Fi Heroin, explore antiquarian equipment but do not to my knowledge have a forum function. The London Jazz Collector is a good site for a self-explanatory topic.
Is there one user generated content site that has it all? Not to my knowledge. One of the reasons a general topic audio board like this one seems chaotic is that it covers a lot of ground. But if there are specific brands or areas of interest, you may find some depth, e.g. Klangfilm. There are, as mentioned upthread, some Facebook (I know) pages dedicated to topics like horn speakers.
One thing to consider is starting your own blog to address the things that interest you. It may attract others. NB- I did this starting in 2015, mainly to talk about LPs and record care-- it does take time and effort but scratched an itch for me and in the process I learned a lot from others.
Good luck,
Bill Hart