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.

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.

The Loki may be a solution but it may also cause you problems. I am making an assumption here, and you may already know this, but the Loki is more than likely a 4 band peaking style e.q. with a very low “Q” so depending on where the center point of each band is you may be boosting or cutting the wrong frequencies for your room. For an example in my room boosting the frequencies between 40 and 50 Hz (approximately 1/3 of an octave) gives me a very full and large sounding kick drum, but if I boost 63 Hz just a little bit (the next 1/3 octave above 50 Hz) than that same kick drum becomes very boomy. My guess is that each channel in the Loki covers a much larger range than 1/3 of an octave. I don’t know what the range is but it wouldn’t surprise me if each channel covered a couple of octaves or more.

Another option that may be worth looking into would be a miniDSP unit. It should be comparable in price to the Loki but I believe it has several bands of parametric e.q. capabilities where you can select the center point of the frequency that you want along with the “Q” so you can better tailor the unit to the needs of your room.