In general "good cause" is such a cause as would, in a similar situation, reasonably motivate the average able-bodied and qualified worker to give up his or her employment with its certain wage rewards in order to enter the ranks of the unemployed. Evenson v. CUIAB (1976).
Thus, under the law, demotion with significant wage reduction is usually not good cause of quitting a job, unless the reduction is so drastic that it reduces the employee's wages in a way that reasonable leaves him know choice but to quit. Further, the law is clear that if you resign before you are laid off or terminated, even when you know for sure you will, you will likely be disqualified from benefits.
It is strongly recommended that you counsult with an experienced employment attorney before you make the decision to quit your job to make sure that you explored all the options available to you under your circumstances to maximize your chances of eligibility for unemployment benefits.