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

 By specs, they don't go low.


My W18E001's dual Thor cabinet go down to 40hz
Just added a  2.2uf  Mundorf SESGO cap to the 10 uf Mundorf Supreme Gold/silver and man, massive bass. 
Caps make a dif.
I would never add a  sub, as my other tech geek mentioned, a sub is  for HT, not  high fidelity.
Change out the caps in the  bass to Mundorf SESGO, and hear what happens. 
They are not cheap, but well worth it. 
I don't believe in the 20-40 hz range , = there is not much there to make any dif. 
Just get the 40-60hz range working at max, and thats all you need.
Now if you really want to go extra bass, replace your 8 inch woofer witha  Seas Excel Graphene W22 at $1k pair. 
= Beats out any sub for hifi. 

Subs, well integrated, can be glorious,

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Read above, 
Just bashed your sub idea.

Almost any speaker can be improved with the addition of a good sub or subs (my opinion).

HT usage, 
For hifi, a  sub will pollute the  soundstage. 

To start, you’re satisfied so theres no need to add a sub!

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Adding a  sub in the OP system, will only make a  unhappy camper, out some $$$$.
Subs are for HT ONLY, 
Subs should havea  label
~~NOT FOR HIFI USE~~~

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.