I know you said without testing, but testing is the fun part! The real challenge in this business is to understand your user base and their tastes. I adore my surfers. They pay my bills. I take my porn offerings seriously. I usually buy a membership to the program I plan to promote.
I have also hosted small parties locally for my surfers. Usually 100 of the more fanatical show up to drink and eat.
I would highly recommend sampling your users with an online questionaire every 3-4 months to get a better idea.
http://www.chumpsoft.com/products/phpq/