Thanks for informing us about this problem with the IQaudio card. Regarding your question on sysdefault and default; sysdefault is in there in case you need more system rights to run the card. There is no other fundamental difference in the two.
Regarding your volume problem, we changed the settings for the IQaudio DAC+ in the Raspberry Settings, so if you update to beta on an new system and directly choose the IQaudio DAC+, the volume will be tuned perfectly. You can also use the sliders in Raspi Settings to make it louder.