2 subwoofers - 1 brand?

Thinking about adding a second subwoofer. I currently have a Piega P-series home theater system with the P-Sub1 subwoofer. People are recommending that I add a second sub to the system for better bass. Someone told me it should be timbre matched to the rest of the system, but I thought that was not true for subwoofers. Any advice would be appreciated.

If you add a 2nd sub, I would do it as a stereo sub which means you will need the same brand and model.