This idea is marked as "not right now" bacuase it involves a 3rd party. Instead please support the similar idea Support USB Audio directly - bypassing the Android driver.
External USB headphone DAC/amps for Android phones are becoming a thing. The release of the LG G5 and its "Hi-Fi Plus module" is an example that has been widely publicised. What Hi-Fi has a Best DACs page that mainly lists portable audiophile kit - a few years ago the DACs in What Hi-Fi were all mains powered and large. I would personally prefer to carry a phone and a DAC rather than a phone and a portable music player mostly because of Spotify.
USB Audio support in Android 5 (Lollypop) is imperfect - see this support thread.
AudioQuest, makers of the Dragonfly, have recommended the USB Audio Player PRO (UAPP) app. The app uses its own USB Audio Driver to bypass the imperfect Android driver. On my phone I get glitching and up-sampling in Spotify with the Dragonfly and the Android driver but there are no issues with UAPP.
Spotify developers could work with the makers of USB Audio Player PRO (UAPP) to support their USB Audio driver in Spotify. I imagine that UAPP would need to be installed before Spotify would see the driver. So I imagine UAPP's developers would help with the integration because it would lead to more sales of their app.
Updated on 2019-08-29
Your idea was submitted a while ago and unfortunately didn't receive the amount of kudos needed to stay active in the ideas board (100 Kudos per year).
In order to keep the Ideas Exchange clearer I'll close this idea for now, but this doesn't mean Spotify rejected this idea.
If you think this is an idea which needs to be implemented, you're welcome to post it in a slightly different form :)
Have a nice day!