Honestly, nothing negative. I bought mine from Apos and they had a free cable upgrade, so I chose the balanced 4.4 termination. I like their Flow cable much more than the stock one. I was using various upper end gaming headsets. Then I dove into the world of iems and had a couple high end sets. My 109 is paired with the Fiio k7. With this setup, there is a fullness to music. The detail and separation is fantastic. I hear things in songs that I never heard before. Songs I’ve been listening to since the 80’s.