I've found that http://mynoise.net offers better sounds for concentration than most music.
Using https://brainaural.com/ with brainwaves at about 13.5 Hz - 14 Hz also seems to help my concentration. (I have no idea whether it's a placebo, tied to habit.)
I haven't tried these sites yet, I'll take a look. Do you use noise-cancelling headphones? The best feature I've found with my QC35 is not so much the noise cancelling but the combination of noise cancelling and audio. Obviously with noise cancelling on, I can hear most of what's going on around me, just a bit dulled. But add in a VERY small amount of quiet music and the rest of the world disappears in a way it doesn't with just NC.. or that requires MUCH louder music without NC.
I do use noise-cancelling headphones. I use some inexpensive Sony headphones that have basic noise cancelling, and they have helped a LOT in the open office.
Using https://brainaural.com/ with brainwaves at about 13.5 Hz - 14 Hz also seems to help my concentration. (I have no idea whether it's a placebo, tied to habit.)