Considerations
You’ll want to take into account your needs in terms of output, your available physical space, and of course your budget.
Kali Subs
Kali currently offers two subs. The WS-12 V2 and the WS-6.2. The smaller WS-6.2 is more than powerful enough to keep up with a pair of IN-8 V2s (one of our more robust monitors). Assuming they're roughly level-matched, the IN-8s will reach the limit of their performance before the WS-6.2 will. For critical listening, mixing, or mastering, the WS-6.2 will be more than sufficient. If you're using the system for playback and you want Entirely Too Much Bass, the WS-12 will give it to you. Again, if you're looking for neutral reference and good translation, you'll get everything you need from the WS-6.2.
Specs
Full spec sheets can be found HERE!