I'm coming from the Samsung S3 before this amazing phone (which makes the S3 seem like a toy!). But the S3 has a great Hi-Fi Wolfson audio chip that could be programmed directly via custom kernels (Boeffla). It had features like DAC Direct, DAC oversampling and FLL tuning for getting great sound out of it. Are there any such kernels for directly programming the Qualcomm DAC on the M8? Also the headphone volume limit could be removed and changed from 53 (stock) to 63 giving a big volume boost and making the volume seriously powerful. Is this a possibility on the M8? I find the factory headphone output a lot quieter on the M8.
If you think it doesn't sound loud enough, try FauxSound. But don't expect the same audio quality compared to stock. Stock sound has more air and sounds more detailed.
Try it yourself.