What is the difference between a custom instrument and a concept bass?

The difference is that a custom or master built bass is designed for and with each individual customer. All the features and details are chosen together with the customer to make a truly unique instrument perfectly tailored to the customer’s specific needs.
A concept bass is designed around a concept (and this is where the name concept basses came from) and is designed by us with just a few options to choose from by the customer. All the features and available options for a concept bass are carefully selected to achieve a desired sound and feel. Each concept bass is developed over a period of several years, by building prototypes that are all tested in studio environments and on the road by pro bassists. We do not introduce a concept bass until until that exact desired sound and feel are achieved.
There is no difference in how a custom and concept bass are built, the quality, size or dimensions. But because we can pre-made some parts for our concept basses (truss rod covers, electronic cavity lids. Body blanks etc) we can offer a concept bass at a slightly more economic price.
You can read more about it on the the Concept Bass pages