Built-In Bottom™ shells have a horizontal outer ply and a vertical inner ply. 
Built-In Bottom™ shells have a horizontal outer ply and a vertical inner ply.
So, what's the difference between VLT and Built-In Bottom? In a nutshell, the optional VLT feature utilizes vertical plies on the inside and outside of the shell, while Built-In Bottom only has vertical plies inside the shell. It provides a more low-end response. While optional on all Collector's Series drums, Built-In Bottom comes standard on Collector's Series bass drums and tom-toms 14" and greater.