Erich, a couple thoughts on how you have the forums set up. You currently have something like 13 subforums. You might only need 1 or 2. A web forum only needs to break itself into multiple subforums when it has so much traffic that topics roll off of the first page of the forum faster than they're read. You have the opposite problem. You have threads that are years old on the first pages of your subforums. It makes the forum look dead, which doesn't keep people around and reading.
While I know you're probably trying to be organized, I think it more cumbersome to navigate the site. Casual visitors just aren't going to surf into every subforum, which means you're getting fewer potential views of threads, and thus less participation. And you also have very redundant categories. I don't think you need two forums for waveguides. Or separate forums with descriptions "Questions and Info on all speaker kits available" and "Questions about kit assembly, picking the right kits, etc" with another forum for "Flatpacks and Assembly".
I think you could combine all nine subforums under General DIY, DIYSG, Denovo, and even GTGs into one subforum. Or maybe one General Audio forum plus one for DIYSG/Denovo specific products, only because it looks like you're trying to split the forum out on its own, away from the DIYSG branding. Don't look to AVS as a model, as it has waaaay more users than you're dealing with for now. AVS probably has more people actively reading at any given point of time than you have registered to post. Even Techtalk only has six subforums, and they probably only need three.
Just an opinion based on my experiences as a guy who's been posting on "internet forums" since the 90s. I know having a strong community will help you a lot, since the more support the community gives, the more time it frees up for you to handle the rest of the business.