[Google Pixel XL Questions & Answers] joesee:
Just ran across your thread here and after about an hour of Googling, I came across something interesting. The HTC uses the Snapdragon 820's DAC. Here's a read directly from Qualcomm's website regarding the sound quality of the Snapdragon 82X series SoC's:

There are many portable Hi-Fi audio devices available to consumers with prices ranging from a couple hundred to thousands of dollars. But before you take out the wallet, here’s a question: Have you ever considered the device that you use every day—you know, your smartphone?

The HTC 10, powered by the Snapdragon 820 processor, features the Qualcomm Aqstic audio codec.

A Qualcomm Snapdragon powered smartphone with integrated Qualcomm Aqstic audio codec is engineered to deliver those sounds—no additional equipment (or dollars) needed. And there has been no shortage of smartphones featuring Qualcomm Aqstic, including the HTC 10 and Xiaomi Mi5, powered by the Snapdragon 820 processor (check out these awesome reviews).

The Qualcomm Aqstic audio codec has an integrated DAC (Digital-to-Analog) converter that supports up to 192-kHz/24-bit playback. This is a high-resolution standard that recording studios use to master tracks—it’s how you produce that pure listening experience that satisfies even the most sophisticated ears.

Many aspects contribute to the ideal audio experience. Once the analog signal is converted by the DAC, it still needs to be amplified for the headphone experience. The Aqstc audio codec is designed to deliver the amplification performance that is usually captured by impressive SNR and THD+N numbers. For audio geeks, SNR (Signal-to-Noise Ratio) and THD+N (Total Harmonic Distortion + N) are specs that determine the dynamic range and signal purity.

With the SNR at 115db on the Aqstic codec, you’re getting the best dynamic range—one where you can feel the dramatic change of volume and emotion of something as beautiful as Beethoven’s Third. Ultra-low THD+N at -105db means that the tone of the instruments and vocals you’re hearing is a faithful reproduction of the recording, and no additional noise or signal is needed to give it the most authentic sound reproduction.

The Aqstic audio codec also supports dual oscillators; the dual-clock design supports sampling frequencies 44.1kHz and 48kHz independently so your music files can be processed at native sampling rates without any conversion.

The Aqstic smart PA (Power Amplifier) is another piece engineered to deliver even more oomph to your external speakers. Your music can be loud and crystal-clear thanks to the wide dynamic range and low noise floor on the smart PA. It also features Speaker Protection that protects your speakers from damage when pushing volumes levels beyond normal range.

Maybe the most impressive part of the Aqstic solution is that it combines the best of both worlds in performance and power consumption. Every aspect of the Aqstic audio solution is highly integrated in the Snapdragon 820, engineered to provide Hi-Fi audio quality while maintaining great battery life.

It sounds like to me that the Pixel should be right on par with the HTC 10. I've been an audiophile for a while now, dating back to the original Galaxy phones with the voodoo awesomeness and Wolfson audio DAC's. I currently own a HTC 0 and for cans I use B&O H6's I bought in Amsterdam and Westone W4r's for IEM's. I also own a Dragonfly Audio v2 external DAC. I can tell you with my personal experience that I LOVED the sound from the V10, while I only like the sound of the HTC. With the firmware tweaks and V4A add ons to the V10 and it's built in ESS Sabre DAC, that thing will give me chills when playing my favorite tracks.

Keep in mind it's personal preference, but I travel the globe and music is my life. I just thought I'd share with you my experience with both the V10 and HTC 10 (effectively the Pixel as it uses the identical DAC)

Also keep in mind anyone that suggests to purchase an external DAC for a solution is silly because you cannot charge the phone and use the External DAC at the same time without yet another dongle that supports it. The V10, you plug in headphones and an external charger and you're good to go.

I personally cannot wait to see the actual audio out from the new v20!

Good luck!

Best regards