It's a forum with 2400 pages. Manually editing ad code for even a substantial portion would take more time than available. And automating it would require a complete re-write of the code (even though it's open source). It's not like Wordpress where you can stuff ads in wherever you like...
Amazon should have worked but their content-based ads are even worse than Adsense. Theoretically a thread about squeegees should produce an ad from Amazon featuring a squeegee that they sell. But as often as not it would be a Kindle or a music DVD.
I like the forum for content... the users are amazing. It's been up for just over 2 years and gets about 700 uniques a day with good page views. They are not always shopping but a surprising number are looking for information about specific products. I put Adsense on it in late January and expect my first check this month.
Focusing on ads seems to me to be counter-productive. Yet there must be something better than Adsense (which, for only 2.5 months, is not doing all that badly). I would rather focus on content and let someone else focus on sales.
You haven't asked for help, is it because you think you know enough? You don't know enough but at least you are here and perhaps you are looking for helping in which case you should be commended. You still have much to learn but you're at a good place to improve your skills.
No reason to do one or the other, it's possible to have both. But be careful because one could hurt the earnings of the other.
One way around it is if you have a forum topic that is very specific to a kind of product. You can then create a template style specific to that forum and modify the php file for viewing topics to include Amazon ads for specific products. A way to quickly catch low lying fruit (less work, natch) is to plow through your forum and manually turn specific product mentions into Amazon encoded links. A third way even lazier way is to do a site:search for amazon (site:example.com amazon) and add your affiliate code to the link.
Advertisers generally piss on forum traffic. Monetizing a forum is the one of the hardest forms of online marketing because of the poor conversions. So if you are not willing to put hard work on it then your forum is not going to reach its potential. Don't be so frickin lazy, it's annoying. I'm trying to slap some reality into you so you do better. Turn around and bend over. You could probably use a good kick too.
sticking on a load of ads and just expecting sales rarely works. you have to try and 'pre-sell' the product, like jimbeetle says. you need to make the user actually want it first. then there is more chance of them clicking the ad and buying it.
if i was in your shoes maybe i would try posting up some reviews every day. pick a product and start a new topic, giving it a big detailed review, with pictures and everything. then put the affiliate link at the end. that will probably have more chance of leading to a sale than just putting a random ad on the page.