Cheesier Cheese Sauce?
The first way to boost the cheese flavor in any cheese sauce is by adding salt.  In Mac + Cheese, authors Allison Arevalo and Erin Wade give the following tip:


  If you have added the proper amount of cheese to your mac, but it still doesn't taste “cheesy” enough, chances are the problem is not cheese, but salt. Salt brings out the flavors of all kinds of food—meat, chocolate, bread—but it is particularly noticeable with mac and cheese. So if your mac isn't tasting as cheesy as you want, don't start grating more cheese. Try adding a bit more salt first, and see if you don’t notice a huge difference.


Another way to boost the savoriness of a cheese sauce is to add blanched or sweated onion puree before adding the cheese.  Many nacho cheese sauces start out with a bare-bones Bechamel, leaving out the onion and veal fat found in the richest Classic Bechamel recipes.  Adding sweated onion puree and cheese to a bare-bone Bechamel you end up with something richer and more savory.  This is the approach to cheese sauce that Michael Ruhlman suggests as a base for cheese souffle and mac and cheese in his book Ruhlman's 20.

Using aged cheeses can add a ton a flavor, but don't bother using anything too expensive some of the more pungent, barnyard flavors in cheeses like English Farmstead Cheddars can become too aggressive in a cheese sauce.  As others have mentioned some aged cheeses are dry and can lead to seizing if not balanced with other ingredients which help to reduce seizing.  One way to avoid seizing when using dry or aged cheeses is to make at least 20% (by weight) of the cheeses you use process cheese like American or cream cheese.

The more "modernist" method of getting aged cheese flavor in a cheese sauce relies on the use of so-called melting salts such as, Sodium citrate and Sodium hexametaphosphate.  These ingredients resist seizing and are used in lieu of a butter/flour roux in cheese sauces.