Passthrough means different things. For DTS it just means do not decode internaly and send out a compressed DTS stream for decoding externally. I don't know what pass-through means for Spotify, if this means send the OGG stream uncompressed I'm not convinced the receiver will be able to decode it if received compressed audio in the OGG format over SPDIF, and certainly all DACs won't be able to. Any Spotify player, be it the original Spotify app or a Kodi plugin, needs to decode the OGG (or maybe AAC depending what Spotify API it is using) stream to 44.1/16 PCM first, and then send it out. This is when bad things can happen. If pass-through means it can send it bypassing the Android audio mixer stack all is good, but if it is using the standard audio output it will be routed through Android audio stack and likely modify. I'd need to try Kodi to confirm how it works. But typically pass-through means do not unfold the signal in the app and send it out as-is, as opposed to bypass the OS mixer.