Hi Aidan!
You can build your own streamer on a raspberry pi with a DIGI+ or DAC+ Or other card, and install plexamp headless on it. You can then control it from the plexamp on your phone, and cast to it, just like the chrome cast. Gapless is supported with this solution.
You then simply connect it to you amplifier via coax (DIGI+ HAT) or line-in (DAC+ HAT).
See info here: Plexamp for Raspberry Pi Release Notes
And for a simple installer, and additional build-information, see here: https://github.com/odinb/bash-plexamp-installer/wiki
