In my car stereo days, we referred to these genercially as "high-to-low-level adapters". Speaker level in. Line level out. Depending on your level of persnicketiness, you can find these at a variety of price ranges.
Another consideration here is that you have a single RCA cable and your amp has left AND right channels. While bass is generally summed to both channels, you can take your pick of left OR right. You may want to look for a "summing" adapter if you want to make sure you capture all the bass information. Probably overthinking this a bit, but just throwing it out there IF it matters to you.
As far as termination on the amp end is concerned, it may depend on you amp. IF your amp has speaker A and B connections, then connecting to speaker B will give you a wide range of connection options. IF your amp has speaker A only, then. you will be joining other speaker connections which may make the connections more challenging. If you are using bananas, for example, adding more bananas is certainly above my pay grade. But, generally, raw wire is best, but takes a little more time a patience to get it right.