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

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.

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.

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.. 

You need at minimum to add a stereo pair to just about any system 

"...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 sorry, first poster. Add an S to the word sub.

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.

You don't need a sub.  If you're satisfied with your system's sound without one, then you've already answered your question.  Now, adding a sub(s) can have a very positive effect on your system, but I would classify that as a want, not a need.  For some people a subwoofer is essential, for others not so much.

Not only do subs add very low notes that the vast majority of ’full range’ speakers aren’t capable of reproducing, but there is a lot of ambient information in low frequencies. Especially in classical music, recorded in a concert hall.

Even if the music itself does not have any very low frequency info, the concert hall does.

If you want to hear a concert hall become bigger and the boundaries better defined, add subs.

Though most speakers will go down to 50hz, the addition of a subwoofer adds thae very low bass notes that a speaker is unable to reproduce.

Do you 'need' it, probably not. But, if you are looking to get every bit of sound you can, then a sub is what you want.
When dialed in properly, as subwoofer offers that 'je ne sais quoi'.

My pick for a sub would be a Vandersteen. It utilizes a crossover that relieves the amp from reproducing the power demanding bass notes, and integrates quite seemlessly.
B

There is low frequency info in a lot of recordings that you are missing if your system cannot go down to 20 Hz. So the answer is you need a sub. I prefer a forward facing sub between the speakers, a stereo pair is better, and stereo towers are best. 

How do you know? Very simple. Count your speakers. If you count less than 4 subs, you need to add subs. At least until you reach 4.

Yes it really is that simple.

(Add "a" sub. Good one. Like I would go for that. Just one sub. Chuckle. Snort.)