HTC recently announced the HTC M8 Harman Kardon edition for Sprint in the USA. The device from the outside looks just like any other M8 and you could be forgiven if you asked what was the big deal until you played some music on the phone. After a failed experiment with Beats Audio, HTC has turned to Harman Kardon to appeal to the music centric community. HTC has teamed with the Harman Kardon folks to tailor the equalizer on the device which affects the sound output both from the Boomsound speakers and the Headphones. At the end of the day it is just another software tweak.
As a result it is possible to port this to your HTC M8 which may not be from Sprint after all. All you need to have is a rooted HTC One M8 and some inner strength to go ahead and flash things. You can easily turn your normal HTC One M8 to a sweet music sounding device by flashing a zip file which has the equaliser in built. You can find the ROM and instructions by heading to the XDA page here.
It is advised that you are on an S-OFF HTC One for the mod to work. Also, it is better if you flash this zip on top of a stock variant of the HTC M8 as it is known to give issues when used on custom ROMs including the GPE ports. Once you have installed the .zip file, you will be able to find the option to turn Harman Kardon equalizer on in the Settings of the device.