The problem with making many subforums is that it results in the defragmentation of communication. Whilst to a regular member it might be quite an attractive proposition, however to a new member it could be either confusing or overly complicated. Plus some of these new sections might not be popular at at all (ERB's for example). Which discourages people on repeat visits, the classifieds sections are successful because they have many new threads in a day. I do not think adding more subforums is the answer.
However if anything i think the search function should be improved either to be modified to allow less then three character searches, or maybe when starting a new thread members could have an icon selection which could have terms such as "5 string" etc. Which can be filtered by clicking a link at the top of the page.
Much like this.
(Yeah i subscribe to interesting sites)
Click on a subject such as "Unfunny" and it leaves just those posts.
So we could have icons such as "5 String" "4 String" "Vintage" "Extended Range" "Weird" etc etc.... It could be entirely possible to use more then one icon in a thread to further help your search.
This option would however be entirely dependent on the current forum technology we use. It might not be implementable.
Just a thought!