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


I like my speakers, I like the SQ of my system, so I'm not asking this question because I'm seeking a remedy to a deficit. I just wonder if it would sound even better with a sub. and I don't want to buy/audition anything based on mild curiosity. Also, like many of us, I don't have an unlimited budget and wouldn't care to stretch it unnecessarily.
How does anyone else decide whether to add a sub or play a pat hand?
My speakers are ATC SC40v2s. By specs, they don't go low. To my ears, the bass is much more satisfying than anything else I've listened to in my limited experience.

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OP here. Thanks for the responses so far. Is there a consensus as to how much one needs to spend to get a decent sub that really makes a difference? I’m not saying I want to spend the bare minimum, I’m just interested in a basic parameter.
"...Is there a consensus as to how much one needs to spend to get a decent sub that really makes a difference?..."

You never want to buy a cheap sub but good affordable subs like HSU Research work really well. 
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..