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

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

An interesting test is to do the measurements in a system that has subs with the test CD and sound level meter and see if it shows too much bass.

enjoy

Minorl: I like your idea of testing DB at all the frequencies. Aside from my wondering about subs, I'm sure I have room problems I don't know about since I have almost no points of reference as to how rooms/systems without problems sound.
It's certainly possible (and maybe probable) that I don't need anything more than what I've got, given that I can maximize/improve without adding devices or components.

By the way, I'm pleased that so many posters are really trying to help me vs promote other agendas or make me feel like a dumbass.

Do the measuring with a sound meter app and white noise to see if/what you are missing or not. Only then will you know for sure. Having that lowest octave reasonably flat in response along with all the rest makes a significant listening difference ranging from huge on many recordings to minor or even none on others.

Room acoustics is a different issue and greatly impacts bass. Not a bad idea to get a handle on that first with what you have before adding anything.

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.

I do not want to be rude "the best way" and then suggesting something that were the best way at the 80-ties. 🤔

Maybe a computer with free software like REW instead of a CD. And a inexpensive AND calibrated measurement microphone ~70-100$ much better than any DB meter..

But a dB meter is better than nothing and it is almost as nothing also in comparison. But I understand that not everyone are handy with computers and software. (No pain no gain.)

You will get graphs with levels and frequency at any point you desire in your room and move your speakers and see exactly what happens for each change you make.

You also get so much more for example can get help to find crossover point and delays for your subwoofers to time align them with your mains automatically generated.

You can do multipoint measurements and also do moving microphone measurements and you can see decay times for across the whole frequency band to see what your treatment do for you... And so much more that you can’t do at all with a DB meter.. so much more so it is not even funny. 😉💖

And remember our hearing is NOT the same as a flat graph.✋ Even if we would like it to be.. it is convenient to think so. This is regarding to get something flat.. it LOOKS nice with a flat line.. anyway microphones measure accurately.. let me explain below.

Study this graphs that show how our hearing works across the frequency band. Note that they are far from horizontal or linear/flat for that matter..
https://en.m.wikipedia.org/wiki/File:Lindos1.svg

If we measure with a microphone the same shape as the graph shows sound coming from a speaker. Then we would experience that speaker as truly flat. With equal loudness from 20 to 20000 Hz.

So as you see in the graph, if we follow the 60 line that is 60 dB at 1 kHz so for us humans we perceive that at 20 Hz at 107 dB is AS LOUD as 60 dB @1 kHz.
Because our hearing is less sensitive at 20 Hz.👂

In other words if you strive for as a goal to make your frequency response flat so you have 60 dB also at 20 Hz then you precive when you listo, that you have attenuated the level at 20 Hz by 47 dB!

And that in it self make us to not be able to hear ANYTHING at all at 60 dB @20Hz! 😱
When you can clearly see that 60 dB @20 Hz is BELOW your hearing threshold (the other read line named the same).

I hope this is food for thoughts. 🤔😍🌷❤️

(This is just science worked on since 1933 and then reassessment and revised in 1956, which became the basis for an ISO 226 standard later on.)

You need a sub if you can not play the music you like with sufficient bass and image size and restricted volume levels if you have any of those problems a sub or larger speakers will fix those issues in most cases.

To answer your question if you need a sub, get a sub with a Bluetooth App. SVS and Paradigm are two of the few offering that feature. With a swipe of your finger you can mute the sub. A/B ing with only a few tracks will let you see the NIGHT and DAY difference. The most important part is you are doing this from the listening position.  No more running behind the sub. A quality sub will fill in the missing frequencies 40-20 Hz or lower, even the most pricey floor standers can’t produce. Check out Paul’s of PS Audio YouTube videos on the subject. Even my Golden Ear Triton 2’s which have powered bass modules built in benefit from my SVS SB-3000!

Just TRY a sub. We're beating a dead horse now.

A video I watched said even a violin sounds better with a sub. That did it. 
I was associating a sub with that annoying thump from a car truck and didn’t understand why anyone would be happy adding that to a 2 channel system. I’m new to better hifi and quickly learned you don’t know what you don’t know. 
Just found out my second sub is on the way. Planning for all four by years end. 
Whatever you do, do it!

I use a pair of subs with my main stereo pile (RELs bought used in great shape...inexpensive...I now own 3 of them) and this does help with the dreaded standing wave issues, but I also experienced a huge improvement when I used just one sub. Note that the world in general, meaning simply the sound of life, has a very wide frequency range...most speakers can cover from maybe mid bass to relatively high frequencies, but subs provide a more life-like element including some ambient sound as well as accurate bass and often make your main speakers sound much better. I check where my main speakers drop off and put the subs close to that point...adjust for taste...I also use a Loki sometimes if a recording sounds like it needs help from Knob Turner, and I have a new version coming from Schiit (back ordered) and it will be interesting to see how they improved the thing.

@optimize mentioned some good points as have some others. To address your question: the sound will definitely be better with a sub or two or more. Even if you have a speaker that can truly reach down to 20Hz the position of your speakers will dictate how the room nodes develop. If you move the speakers to a position where the bass is good it is extremely unlikely that you will be happy with the mids and tops!

A speaker that produces from 40Hz and upwards with 3 subs optimally positioned will easily outperform  the necessarily huge and expensive full range speakers.
You can also rest assured that all domestic rooms will need some acoustic treatment which is another issue.

Along with some good advice I see the inevitable recommendation to add tone controls or room EQ. which will not sort out the unavoidable peaks and nulls. A 40Hz tone has a wavelength of about 28ft which when bouncing around will combine constructively (peaks) and destructively (nulls), using room correction the peaks can be attenuated so they are less offensive at the listening position but when you move around you get to hear all the peaks that remain. With nulls, which is musical information that is missing, understand that no device can manufacture that info, it's gone AWOL. Trying to pump lots of power into that frequency will just cancel with the same power. It's futile and if you drive the amp into clipping you stand a good chance of blowing your tweeters.

EQ can work once the Schroeder frequencies are taken care of which varies from room to room, about 300Hz average. How to take care of them? Add bass traps and multiple subs and as has been mentioned, get a mic.and free program off the net. If you go this route  you will be utterly amazed and will have little desire for any EQ!