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

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.

The ATC SCM40s have low frequency extension as good as any three-way speaker of their size and better quality bass than than nearly all at that price point. I know that model. The specifications ATC provides is a bit misleading because their speakers are tested or the specs published differently than others.

I too would recommend an audiokinesis system if you want to go down that route. I have heard it. It is very good, affordable and lends itself to setup success rather than frustration. Otherwise a sub would be way down on my list of rabbit holes.