I am thinking about buying these from amazon for $30. What I am worried about is the response range. Shouldn’t a sub go bellow 48 Hz? Is the response range just for the two main speakers? Is there anything else I should be worried about with these speakers?
I wouldn't if I were you. I went through two different sets of cheap Logitech speakers that sounded great but only lasted a few months at the most until I started having lots of miscellaneous problems that made them unusable. Logitech cuts corners and makes cheap crap that I wouldn't get unless I needed a quick and temporary replacement.
They have really good reviews on amazon. Some people seem to be having bad luck, but 4.5 stars over all makes me feel pretty confident.
A sub should go down past 30hz. However for $30 you aren't going to get anything like that. $30 is good if you NEED speakers, and have no money. If you want something that sounds good, then reserve $500 or so. Speakers aren't like other PC hardware that go obsolee. Speakers should be a one time purchase, and because of that I generally recommend people spend more than $500 so that they are satisfied with what they bought for the next few decades.