Will a subwoofer add depth and clarity to my system, or just bass?

Yes...not that difficult folks.  REL gives a very musical result.  Move it around till you hear best bass integration.  Try even with plane of speaker mid/bass drivers facing parallel or horizontal.  

Not true plga.

Plga, are you using the QC 500’s yet?  If so, you are not getting foundational bass response...you may be pleased with what you are hearing but 41hz @ -6db is not room shaking.  Stereophile even mentioned that a good sub would be beneficial.

So when ya ordering that sub plga? ;))

Glad your happy plga...better off not knowing sometimes anyway.  Saves money!

All good!  Experimenting is fun...enjoy and happy listening.

Hey Sparky, I’ve owned Wilson 6’s, 7’s, Dynaudio C4’s, B&W 802D3’s etc and Reference hear from ARC, Levinson, Krell etc..  Been there and done the whole no sub thing.  Short of those types of systems and great rooms (insert plga) a sub adds texture, spaciousness and warmth.  It is not to blast more bass.  Adding room correction electronics to a system, IMO adds extra artifacts and strays from the original musical content.  Talk about using a computer to deal with a problem an abacus could solve...jeez!  Placing a sub is hard if your hard of hearing.

Depends on room, budget and goals really.  What is the current setup, wish list and desires for sonic bliss...musical tastes etc..

Ya puts the speakers and the sub in da room and ya moves em around till day sounds da best!  Badabing....hopefully not badaboom :))

No such thing as perfect bass response...varies by venue!  So, that being the case, one can relax and get 1 or 2 subs for their system and play around until your happy with the sound.  Doesn’t need to be over engineered or killed with angst!  Have fun, grab a brew and enjoy the tunes.