Here are the least expensive, powered subwoofers that I could find with balanced I/O and a proper crossover (low-passed sub and high-passed mains):
-Presonus Sub 8BT (200€): variable LPF & fixed 80Hz HPF
-ESI Aktiv 10S (240€): variable LPF & fixed 100Hz HPF
-KRK S8.4 (300€): selectable Xo (LPF&HPF) 70/80/90/100Hz
You'd have to look into the quality of which yourself.
Thank you for the feedback, ultimately I decided to return my Kali's and test out the Adam TV5's. While there the salesmen recommended the Adam T10S Subwoofer to pair with as it has the balanced in/outs with the filter's. I was hesitant as the cost exceeded my budget but he agreed to apply 15% off which is $60 cheaper than anywhere else. Factoring in the refund difference from my Kali's the net cost difference to go to this 2.1 setup is about $310. I am waiting for the subwoofer to arrive and can then provide feedback. One big advantage to these adams is since the height is lower than the Kali's, I can angle the speaker at more of an upward tilt on my desk to reduce resonance and line up the tweeters better on my desk.
After first impressions, the resonance of course is still present, but slightly less fatiguing, I am assuming this may just be slightly less present due to the smaller woofer? Maybe because of the lower height and being able to angle higher with my foam pads? Either way it's still an issue I'll have to mitigate in the future but will maybe have to be reduced with EQ since I can't modify too much in my office and I am not sure anything I do with the walls can help the desk resonance.
Just as an FYI I have been using this :
as my test since this guys voice falls within whatever frequency causes the boxiness.
That being said I prefer the smaller footprint of the AdamTV5's to the Kali for my office desk and aesthetically at least, it's a bit more handsome in the room.
Nuance wise, I can notice the cymbals and higher frequencies a bit more, but it doesn't bother me. I wouldn't say I prefer one over the other in that regard. One interesting thing to note, the Adam TV5's have much less volume at 0 gain compared to the Kali LP6V2's. For 90% of my listening pattern, this is more than adequate volume.
I will update when I have the subwoofer and any changes made to eliminate resonance.