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

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

Subs, well integrated, can be glorious, reduce coloration and increase the sense of scale (physical image width/depth) as well as dynamic range. The problem is always, always always getting it well integrated.

I've been here long enough to stop trying to help individuals do this and recommend if you want to go this route  you rely on automatic room correction and experts with the room treatment.

I wrote about this here:


https://speakermakersjourney.blogspot.com/2020/04/how-to-not-buy-subwoofer.html

Erik: Thank you so much. It was very helpful.

 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.