14awg is the MINIMUM you can do for a 15A circuit. If it's a long run, then 12awg is recommended. That is based on electrical code requirements.
However, there is a definite difference in current capability between 14awg and 12awg romex, even on low current source components. I've tested this in my room when I tried 14awg romex to my preamp/processor. It just did not sound as open and the sound did not hit with as much authority as when I did a 10awg romex.
You can use 8awg, but you are only going to get stranded wire and you will need to run this in conduit (per electrical codes). Only romex can be run bare through walls and in ceiling/attic. I would rather have solid-core 10awg or 12awg romex than stranded 8awg (but there are others with different opinions).