Even with it's JBL premium audio, the Prius Prime's sound system is lacking in terms of bass. The 6.5" front door speakers are intended to be the subwoofer but the bass reproduction is definitely lacking for some drivers that are used to better OEM premium audio options. On some of Toyota's SUV's you'll find a dedicated subwoofer in the trunk but perhaps due to packaging the Prius did not get this unit.
Our customer didn't want to sacrifice trunk space but wanted better bass for his car's sound system. We recommended the JVC CW-DRA8 which we've installed in a number of vehicles. The install is straightforward with power coming from the fuse box or 12V outlet and high level input is not an issue with these subwoofers. For compact enclosures like this, we recommend something with at least a 8" woofer as the tone is much deeper than what factory speakers can typically reproduce. We've tried smaller units before and the sound just doesn't compare.
Another nice thing about the JVC 8" compact subwoofer is that it comes with a remote that lets you change the volume, crossover, and phase. We mounted this remote on the driver's side near the shift lever. The subwoofer is small enough to mount under the passenger seat, under and between the seat rails. This subwoofer can output 150W RMS which is more than most factory compact subwoofers.