If you are not getting any bass at all from your speakers then the woofers in them may be blown.
If the speakers have bi-wire terminals there should be a connection between the two plus and the two minus terminals. If you don't have that in place and the wires from the receiver are connected to the top terminals you won't get any bass.
If the problem is that you aren't getting enough bass then you can get a subwoofer with speaker level inputs or use a speaker to line level converter with any subwoofer with left and right line level inputs.