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

With all the speakers I auditioned before buying, I tried both with and without a sub. With some speakers -- big towers like the Focal 948 and Martin Logan ML 60 Xti, the single large driver REL sub I had didn't do all that much, not even for imaging, etc. If I had had a sub which could really reach down there -- 14 hz or 20 hz, then what @simonmoon said about atmospherics would probably have become evident.

That said, every other speaker I listened to was improved by a single sub. Bass notes appeared, image and soundstage enlarged and got more definite, and (most surprising) the tonal richness of mids and upper frequencies were altered in a good way. I'm now on my way to stereo subs before, one day, going with a swarm.

As for price, I think there are good options out there and some have been listed already. I think you could likely get set up pretty nicely for under $2k for two.

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.

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.