As soon as I got my SD1 (june 2001) I too noticed these problems and forwarded them to Ketron :

HISS of the MIC preamp
'extra' (but unwanted) noise while using the Chorus DSP, e.g. Drawbars, E-piano etc.

My SD1 has NO 'pops', this can be caused by external equipment/cables.

I never use the MIC-inputs and it takes a little time to get used to the DSP-noise

Roel