[greglepore]

Its complicated. Subs usually have a choice between line level input and preamp level input, like the expensive sunfire sub above. Even the cheap ones. Yours was probably part of a set, so you don't know which. The outputs of your receiver labeled sub are preamp level.
If you power a pre level input with speaker output, you might damage the preamp in the sub if it has one.
The speaker you have likely has a line level input, which means it may not produce any volume with the above adaptor-your sub out is pre level and the power would be too low. . However, if the sub input is pre level, that adaptor would work.
Safest course would be to try the adaptor and if no volume, assume the sub input is line level. Bass at sub frequency is non directional, so just drive it off of one speaker channel.