I used disable_audio_dither only, the other one I have not used yet.
the "hiss" that I encountered came from several sources, the main causes were the amplifier, the audio cable and the power supply. the USB wireless mouse also contributed some to the "hiss".
1. setting disable_audio_dither removed some of the hiss, but not all
2. changing the amplifier (then a DIY 2W [1W+1W] IC amplifier) to PAM8403 reduced the hiss better
3. changing the audio cable to a shielded one, and clamping ferrite cores on both ends, the hiss was further attenuated
4. separated the power source of the amplifier and the RPi, ....better.....
5. clamped a bigger ferrite core on the microUSB cable, placed it very near the microUSB port, .....much better.....
however, now I use an Audio DAC, connected to a more powerful stereo amplifier (in parallel with several DIY VU meter projects, including ATmega32 based VU meters). Microprocessor based VU meters are known to make the audio line "noisy" with hum and hiss, but not in mine.....
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"
Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"