XLR-XLR cables are traditionally sold as "microphone cables", but in fact they are perfectly fine for high-level signals for equipment interconnects and, as in this case, line outputs feeding powered monitors. Don't put loudspeaker currents through them, though!

Why not make your own?

XLR is a great connector and very robust. It's designed for mic/line level, but why not use it for speaker level? Just use thicker wire!

Become good buddies with your soldering iron.

I'm not familiar with the Samson A8 monitors but from a quick Google search it looks like they are active monitors. Is that correct? If so, you are absolutely fine using a standard XLR mic cable between your sub and the monitors...assuming the signal out from your sub is NOT an amplified signal but is a standard line level signal.

If the A8 monitors are passive and you have an external amp and you are connecting the amp to the speakers then you need speaker cable (it is a different gauge and resistence than a mic cable). In this scenario you would use a standard mic/line XLR cable from the sub to the amp and speaker cable from the amp to the speakers.

Production Sound Mixing for TV, Film, and Commercials.
http://www.matthewfreed.com