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

Forget the Loki and at least try a pair of subs...subs do so much more than only deep bass for a system...then decide exactly what configuration works best for you...

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

short answer is you don't

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. 

Two potential issues with boosting bass levels alone versus adding a good powered sub and setting it up properly:

1) You might increase bass SPL FBOFW but not at the lowest octaves down to 20hz which is likely what is mostly missing to start in most cases. That depends on speakers and room size/acoustics.

2) increasing bass levels will make your existing amp work harder. The lower the frequency, the harder it will work. It may or may not be up to the task so results could well be far from optimal, possibly worse overall. Powered subs deliver a lot of power typically for a reason.....it is needed for low bass. Power demands increase exponentially as frequency decreases. A powered sub will truly supplement your system’s bass capabilities. How well depends on picking the right sub up to the task with your mains and in your room and then setting up up well, which is not hard. Setting up well requires only a free sound level app on a smartphone and a source of white noise which can be streamed from sources over the internet. Or you can do it by ear which is not as reliable for best results but may still deliver the results you are looking for.

First of all, a lot of bass in some music is entirely unrealistic.  I've been with many people listening to recorded music where some said bass was great and others including myself said the bass was over emphasized.

Way too much.

So the best way was suggested before, get a test CD or album that plays pink noise or frequencies from zero to 20 kHz and a pretty good DB meter.  have the meter at the listener's level and measure the readings over the entire frequency range.

See where the gaps, valley, peaks are.  This also helps tell you the room effects and helps solve room issues.

if your system's bass response is what it is suppose to be, IOW, is it flat?, then no, you do not need subs at all.  your system and speakers are doing their job.

If it isn't flat, that does not automatically mean you need subs, it may be the room.  Or a combination of room and system.  

It is definitely not a simple as "I need subs".

A test CD and a sound level meter are not expensive.

enjoy