Long-time D'Addario endorser here. The new NYXL bass strings are the 3rd "prototype" string I have tested from the company. The NYXL's are the first of those 3 that I love! (Stainless steel rounds not being 'my thing' & I don't personally need the whole balanced tension thing.)
Straight out of the package the NYXL's seemed a little bright, but not moreso than traditional XL's. They did break in much quicker though! I was happy with where they settled after about an hour of playing time. (Most strings take at least 3x that before I'm happy.)
Tuning stability is definitely improved. I'm a compulsive show-tuner. I check tuning on a pedal tuner about every 2 songs on a gig! (I know, I know, that's entirely unnecessary. I'm a stickler for pitch is all...) The NYXL's just refused to drift! Very impressive.
The tension might be slightly higher. However, the NYXL's certainly bend easier at the same time. ? It's almost like they 'give' when really pulled hard but have better tensile 'memory' when released.
The selling point for me, though, is more output! I was already familiar with NYXL guitar strings so I expected this. I set up a recording session before swapping a new set of XL's with the NYXL's. The output volume increase was instantly noticeable and I hear more upper mids.
So far so good! (I've only been using them for a week.) For my basses that I tend to use for rock, the NYXL's are a likely go-to string for me. EXP coated XL's are on many of my basses now as well as traditional XL's, with Chromes on a few specialty basses. I see NYXL's fitting into a very useful niche in my tone arsenal.