Consumer Reports? Granted it's premium content, but I would imagine their reviews would include more than just frequency response.
http://www.consumerreports.org/cro/headphones.htm http://www.consumerreports.org/cro/headphones/buying-guide
http://www.consumerreports.org/cro/headphones.htm http://www.consumerreports.org/cro/headphones/buying-guide
If you're ramen profitable and sharing an apartment with 8 other entrepreneurs, keep in mind that many libraries have online access to subscription resources that can include both CR (and other publications) and a variety of online learning resources like Lynda.com.