Looking to Replace Broken Subwoofer on 06 Sienna with JBL Sound System

I'm fairly new to subs and speakers so I am trying to figure out the correct ohm and rms power for the new sub. I'm not looking to replace the amp and upgrade the whole system, but am willing to spend more on a sub if it will have a noticeable impact on quality or loudness. At the moment I know I need an 8 inch sub and that the original amp doesn't have a very high power output, but that's about it. I've been looking at a few 4 ohm dvc subs but I'm not sure if that is the correct impedance and their power seems to be overkill for the current amp. Overall I just want a smooth working amp that doesn't fry any of the electronics, any increase in quality from the old sub would just be an added bonus. Thanks.