What should I set my bitrate at when using on board with hd 558s?
It depends on what content you have. Also not that it's separate for inputs and outputs.
If all you have is CD's or downloaded music, stick to 16-bit 44.1Khz
If you have content that's either a higher bitrate or sample rate, then set it to match the content. Other content will be upsampled (which is not bad. Downsampling is evil though)
If you're not sure what type of content you have, download and install audacity. It'll tell you. I also use foobar2000 as my audio player.
I digitise my growing record collection so I use 24-bit 96KHz for playback and recording.
Digitizing from what? If form CD's then no point recording them at 24 bit 96KHz, you won't get any better quality than the quality of a CD.