I thought that Desaix pronounced his name with the "x" pronounced like in English: "ecks"?
I've never heard Desaix's name pronounced, but in my experience "x" at the end of a word in French is always silent, but Aix by itself is an exception, I think. Perhaps because without sounding the "x" it sounds like "et", or "est", or "ai".
Desay should be pronounced Duh-Zay
Dessaix Duh-Secks; interestingly, I am pretty sure this led a lifetime of puns on him with people wondering how well endowed he was, as it does sound exactly like Deux Sexes (two dicks, you might forgive my French here)
The silent X is true for plural substantives, usually not in place or family names (as you rightly pointed out for Aix); bare Desaix which normally should be pronounced duh-zecks; but again, the French language is full of exception to the rule(s)
