How do I know if I need a sub woofer?

I listen to a lot of old jazz, a lot of it on records recorded and made 50 or 60 years ago.  Mono, early stereo as well as re-issues.  My main speakers go low enough I reasoned.  Besides, a lot of those records had to have the lows cut off so the crappy arms of the day wouldn’t jump right out of the record grooves.  I figured I didn’t need subs.  Paul McGowan kept harping about them, though, and my dealer agreed so finally I sprung for a pair of REL 212/SX.  Keep in mind now there is no information on most of the old records I like to listen to.  The RELs are set up so you would never know they were on...until you turn them off.  All of a sudden the room collapses.  It is like stepping from a big open space into a closet.  Of course I had to try listening to things with some real bass too.  And lo and behold the rafters do shake when the spiffy material is on the turntable.  But that is not the point at all.  It is all about opening up the space.  One other thing I have to comment on without wanting to pick a fight with anyone.  I was in the high end hifi business for almost a decade while I was in college and part time for a few years after graduation.  I have attended a lot of hifi shows.  Personally, I have never heard a good sounding anything at any show ever.  

And yes, I agree it may be hard to provide a coherent presentation when adding subs, I tired for years and absolutely could not achieve the kind of coherence I was seeking. I was and am extremely sensitive to coherence to the point I had to quit three way speakers for two way and single drivers for nearly a decade. Going back to three ways via present Khorns, took me years of listening and mods to bring these up to my requirements. Adding a pair of REL's really completed the picture,  with careful setup I've been able to achieve wonderful coherence. 

Congratulations! I admire your persistence. I’m guessing you added a pair of T/5s (33 Hz)? I tried to make a single T/5x work in both my larger living room and a moderately sized master bedroom. Yes, the experience was better than with SVS subs in terms of coherence, pace, and texture. The sense of spatial cues was impressive, and I even told a colleague I wouldn’t go back to the pre-subwoofer era.

Nevertheless, when the pace of the music picked up, the sub couldn’t quite keep up with the beat. My Buchardt S400 MkII, in a moderate space, delivered more articulate bass notes down to 35 Hz (-3 dB) effortlessly, even more so than with the subs. The Linton’s bass notes were even more nimble and agile. Of course, the definition of ‘coherence’ is subjective and varies from person to person. I enjoyed the augmented soundstage, but the trade-off in pace / texture held me back.

At this point, with an enhanced soundstage from upgrading my digital front end—including streamer, DAC, and cabling—I honestly don’t miss the subwoofers. I feel I’ve already achieved what they offered. The extra reverberation and lingering decay from subs no longer appeal to me; I now prefer a similar soundstage with pinpoint instrument separation and clearly defined imaging.

Well, to sub or not to sub—that’s the question with no definite answer. I can see why many people prefer having one(s), but I also understand others, myself included, who feel no real need for it.  The last thing you should do in this civilized community is castigate people for having legitimate differences of opinion.  You probably know who I am talking about.

The other question would be, should I get sub(s) or use that money to get better speakers...

Pair of REL T9x, a single sub has never worked for me.

Understood.  Empirical evidence has suggested dual works better than one in terms of better elimination of nulls caused by room mode but it does not resolve the pace issue.