A few random thoughts here from a new poster...
The way things are currently structured, there is very little reason to upgrade from Registered Plus to the Bronze/Silver/Gold levels. When I initially registered, it was worth the three dollars not to have to wait two weeks in order to have an avatar and signature, but once you hit Registered Plus, the direct value of subscribing decreases immediately.
Now, that doesn't mean there isn't good reason to donate money to assist in keeping a site running that one is has invested their time and energy in and derives enjoyment from. On contrary, the ability of TOL to quickly raise $3.5k in less than 2 weeks shows that it has an enormous amount of goodwill from a very dedicated userbase. I've seen considerably larger forums unable to raise anywhere near that amount of money in such a short period of time.
What it might be considered that this means is that is that you would be able to maximize the income potential of the site by getting rid of the subscription concept entirely and moving to either semi-annual or quarterly drives for donations.
Conversely, if you want to push more people towards subscriptions, you might consider restricting more server intensive functions such as search and "who's online" to subscriber accounts.
At the same time, you might consider opening up certain things which are normally restricted for a fee. For example, you typically restrict the posting of links to another site or forum, but for a fee an additional field could be made available in the poster info for such links. This is a well-designed, optimized site that ranks quite highly compared to similar sites so it's understandable you don't want people trading off of that ranking for free, but leveraging that page ranking into a way to generate income would be something entirely different.
Another thing you might consider is that you have a handful of fairly active social groups. The activity generated by these groups is to some extent wasted as it doesn't increase your boards stats and will only be viewed by a handful of people, you might find that some of these groups would be willing to pay an decent sized annual fee for their own sub-forum. This would both put more money into TOL from that fee as well as making the topics more likely to be viewed and thereby increase ad revenue derived from them as well.
Of course, for all I know all of these things have previously been considered and rejected for perfectly good reasons. But I figured I'd throw my 2cents out there in case it might be of any help.