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

I'm in the russ69 camp ALL the way. I like columns.

Second they don't have to be powerful subs. They just need to be well made, if your doing low riders (under 24"tall) a pair of 10" HE reference drivers go a LOG ways too.

I'll give you a "Sonic NUGGET" that is hard to beat. It's my go to driver for 17 hz to 300 hz. 90+ % efficient.

https://www.parts-express.com/Dayton-Audio-RSS265HO-44-10-Reference-HO-DVC-Subwoofer-295-463

600-800 watt drivers, they are VERY low distortion 2 is plenty ONE  per enclosure. They work well every way I've tried them. Sealed, ported, multi, and passive radiators.. Just a GREAT driver..

You should hear 4 in a column 72" tall tuned with  a pair of 12" HE PRs.
I did TWO... 8 CF enclosures. Smooth as silk..

It pressures a room so easy and evenly. I use a 2496 active OX full blown Active DSP with on the fly correction from a laptop.. 300.00 dollars
NOT expensive.. an amp 3-500 for 12K.

What kind of budget is OK? There are 1800-2500.00 DBA kits.. James Romney (sp) and a Guy that is on AG.. nice enough fellow. Answer any questions.. 

If you don't try at least one sub, you will never know what you're missing. I think at least 2 subs are needed for a decent stereo, but TRY a good one that goes down to 20Hz., you'll be impressed for sure.

Hey M6, Bruce here... the reason why you have your ATC’s (your welcome by the way!).

Sooo, interesting responses. Conflicting comments here, yet all true.

You’re lucky in that everyone who answered is knowledgeable and provided excellent advice already. 
To start, you’re satisfied so theres no need to add a sub!  Just enjoy your system 😁

BUTT (yes, its a big butt)...

there is more to be had good Sir!  Add at least a pair of awesome subs (think SVS SB 2000 pro or higher or REL T7x or higher, or equivalent used.. at least 2 identical subs, 4 is better, 6 better, get the idea?)

Anywho, three elements to music.  Treble, mids and bass.  No single element is more important than the other no matter your perceived preference or musical tastes.  Bass  (its not base by the way) provides the foundation of the music and will clean up the mids and highs while expanding the soundstage, if it doesn’t drive you crazy in the pursuit therof.

Hey, Bruce. I am eternally grateful. I won’t forget you.
I was researching subs and decided that if I gathered enough extra bucks, I’d start with a used SVS PB3000 and take it from there. We have a large, cathedral ceilinged, complex room or I’d aim at something smaller. So I appreciate your right on the mark advice. You haven’t been wrong yet.
marc