How to connect a single hdmi cable to a monitor AND an amp?
HDMI amplifiers, DACs, and audio extractors (which give a clean digital bitstream on SPDIF) provide an HDMI output. The output HDMI will continue to carry the video portion of the signal and usually (if not always) the audio stream as well.
You can connect the RPI to a dac/amp with HDMI input or a digital audio extractor. The best quality will be found by connecting to an HDMI to SPDIF extractor and then running that into a good quality DAC.
The nice thing about this solution is that you're not dependent on the horrible and CPU-sucking USB offered by the RPI.