Cooking sirloin steaks with Anova sous vide gave me leather tough steaks

Sous vide not only keeps the proteins (main component) at a lower temperature, but also the spices/oils/flavorings. Heat helps release the taste and aroma of all the ingredients, and the sous vide might not be hot enough to do that.

So, while it kind of kills the simplicity, you could always sautee the seasonings in oil before adding them to the sous vide bag (not until cooler though). Or just keep them separate and recombine for serving. Sous vide itself (not the vacuum part) is mainly for maintaining/achieving a controlled internal temperature, and skilled cooks don't only sous vide a dish; they might sous vide and then finish under a broiler for texture, or with a torch. If the sauce isn't working inside the bag, make it work outside the bag.

Alternately, put the ingredients in the bag the day before, and let them marinate overnight in the fridge. The vacuum action is essentially just a speeded up marinade, so this should do the same.

Also, make sure you're using enough (but not too much) SALT.

Finally, call the Sous Vide Supreme company. They know their product's capabilities and limitations and might have some good recommendations.

Just my guesses.

Anything you salt will firm up in texture over a period of time. I suspect that since you cooked these with seasonings and then chilled and left them in the fridge before reheating an eating they firmed up a great deal in the fridge.

If you check out this blind tasting conducted by Dave Arnold at Cooking Issues you'll find some more detailed info about this topic. The gist of it is that for cook-chill-reheat purposes you shouldn't salt the meat before searing. For cook-direct serve meals your just fine doing it that way.

http://www.cookingissues.com/2011/10/12/to-salt-or-not-to-salt-thats-the-searing-question/