What do you guys think of adding a subwoofer to this system?

I've had sub(s) for many years now and highly recommend them.

Having said that, sometimes I've found it really difficult to integrate them to the room.  I purchased a miniDSK UMIK-1 calibration microphone and the REW (Room EQ Wizard) software to help integrate the subs with the room and full-range speakers.

In our last house, my listening room had terrible bass acoustics, eventually I had to install 10 bass traps to tame the extraneous bass the room added to the music, and could have used more, but my wife was referring to that room as my "padded cell".

We moved to a new home and I had a customized listening room done as part of the build and I'm loving life in the new digs!  This room has some sloping intersections of the walls & ceilings (a good thing in my case).

I have a pair of JL Audio F113V2's (otherwise known as "overkill") and one of the sub's amps died, and while the amp was being repaired by JL Audio I was not a happy camper, as I not only missed the little bit of bass augmentation, I mainly missed the "visceral" experience that the subs add to the music.

@mijostyn +1. You nailed it, as did the @ejr1953  using calibration mic and software

I finally sold a 10+ YO pair of SVS powered subs; an Ultra and a Plus (the tall round cylinders). I was never able to get a seamless blend, mostly due to the volume control settings being too coarse, virtually impossible to dial in. In fairness they were not designed for quality 2 channel systems. May be better now, but...

One of the biggest issues I had was because polarity on many discs is out (especially with greatest hits, or multi- recording studios) compared to the rest of the frequencies, and while they have phase/polarity adjustment, it is manual. IMHO, having remote control over volume and polarity is essential to get seamless blending. IMO to achieve great sub bass, you need a separate beefy amp + active XO=$$$$+ SIGH

@tweak1 , I am a fan of passive subwoofers. Most of the amps included with subwoofers are of inferior quality and they interfere with the rigidity of the enclosure. All of the digital preamps that include bass management have separate level controls for all four channels and they all operate by remote. Mine is unique in that I can change crossover frequencies and slopes by remote, very useful.

I too am not a fan of built in woofers, or powered subs. I am currently using Emerald Physics 3.4s (open baffle), which have a 12" mid-bass with 1" concentric polyester tweeter. These would be sufficient in most rooms, but mine is ~35 x 21 with an 12’ open beam vaulted ceiling; front and rear walls are mostly glass. I am long awaiting Emerald Physics 2.8s (open baffle), same 12" mid-bass with 1" concentric polyester tweeter, plus a total of 4 @ 15" carbon fiber woofers. Should they ever arrive, they will present a new integration challenge. Im hoping the OB will mitigate the cabinet resonance issues inherent in most/all box speakers.