How do you know if you need to add a sub (without auditioning one, I mean)?

Depends on what kind of music you listen to and how loud you play it. If you are listening to bass driven music like blues and rock a sub is almost essential, other genres, not so much. The biggest issue with subs is integrating them. Integration is a 2-way process in that you need to integrate them to the room as well as your system. There are tools out there that make the process much easier but most 2 ch. systems are not designed to allow their incorporation.

Multiple subs definitely makes the integration task much easier. I have never heard an actual Audiokinesis "Swarm" bass system, but what I have read about it I believe it is probably the best bang for the buck sub system available and probably one of the easier systems to integrate since I believe the Dayton amp that comes with the system has both low and high pass filter capabilities, which in my opinion is essential for proper integration. Other good value subs I'm familiar with are SVS and Rythmik and HSU is probably worth looking at also. For a little more money the "e" series subs from JL Audio would be worth a look since they also have low and high filter capabilities.

In response to the question below, I run my mains full range and use no room correction.

As for "needing" a sub, well, I am fine with the word "need." Because it's not just about the notes down below. My room was as dialed in as it could possibly be, and I still had some peaks and dips well above the sub range *until* a sub was added, crossed over most effectively around 100 hz. Somehow, that addition changed peaks and nulls in the SPL all the way up to 300 hz. Whatever additional bass I was hearing, it was the effect on other problematic aspects of my room response which solidified for me the use of subs.

So what does anyone think of the idea of using a Loki as a very inexpensive experiment to see if more bass even suits me. I know it won’t be the lower register bass and there are plenty of other differences between this and a sub. Nonetheless, I still wonder what someone more expert than I thinks about this idea.
Along those same lines, if someone has a loki with speakers like mine or similar speakers, what sort of Settings did you end up with? Do you adjust them enough to justify having the Loki? Where is the best bang for the buck in terms of the band adjustment?
As usual, I have a suspicion that a lot of these questions are naïve, born of ignorance, or perhaps just unanswerable. It does no harm to ask, though.

A Loki does a nice job of boosting bass and also taming treble or bringing midrange more forward. It won't be money wasted even if you go on to get subs, and they sell pretty quickly, used, if you want to put it back out there. Low risk, medium reward experiment. It's a good idea.

You don't NEED a sub, but you may want one. Suggest that you buy a Hsu VTF 15H Mk 2 in plain black for under $1,000 (costs another $150 fro wood veneer finish) and see how you like it. Strongly advise that you add a second one when you can afford it.