Cooking sirloin steaks with Anova sous vide gave me leather tough steaks
If you think of tenderness on a scale, sirloin and tenderloin are nearly on opposite ends of the scale. Sirloin, in general is not a tender cut of meat.  Your result has little to do with seasoning or location in the water bath...or even cooking method. Sirloin is a lean and tough cut.  Often sous vide can be used to make tough cuts of meat more tender, but sirloin doesn't really have the collagen or connective tissue that will break down like, say, a short rib.  If you like the tenderness of a tenderloin, you will never match it with a sirloin.  Don't blame your circulator, it is akin to blaming your stove when things don't go well there.