I agree, having the choice of how many posts per page would be good.
Currently, I count 15 posts per page. 20 threads are shown on the "All New Posts" page.
I'm guessing that the site's metrics (number of clicks, number of page views etc.) are greater when posts per page are set to 15 as opposed to 20. So the site's metrics, and therefore site rankings and advertising rates, are affected by this feature.
So how about a compromise of sorts? Allow users to choose between 15, 20, or 25 posts per page, with default of 15. Many users are not going to bother changing the settings, so they'll remain at 15. Those of us who care about this will set it to 20 or 25, and we all pledge to visit Head-Fi 25% more often than before and click on one extra ad every three days. Would this be good enough?