Wow. This is a very thoughtful and complete review. You and I agree on the excellent tuning of KZ's VXS so I was glad to find this ESX review today. My ESX arrived today and my first reaction was positive. Good tuning, very efficient so good to pair with BT sources. But then I tried a sine sweep and found that my set has very little power handling in the subbass. My set produces more harmonic distortion than fundamental at 30hz just over over moderate volume. I can't hear it with music but it's obvious with signal generator sources. It's not current shortfall since my 1AMP has plenty of current for these sensitive IEMS. It's both drivers, not just one. But I wonder if my set is an outlier. Can you try your set with a 30hz sine wave, then increase the volume and let me know what you experience?