HiFiMan HE400S with Focus Pad Measurements The stock pads of the HE400S are leaky and don't have a good deal. They look like this: With the stock pads, there's a bit of upper bass warmth but with a slow roll-off toward the bass extremes. See below. Figure 1: Frequency Response of HE400S with Stock Pads. The Focus pads (below) are absolutely crucial for getting the best from the HE400S. With the Focus pads in place, we get much better bass extension and accuracy. Figure 2: Frequency Response of HE400S with the Focus Pads.