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.

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~~~

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.