key, to be in charge, pipe
筦 centers on a tool for control or flow: a key that opens or locks, a pipe that channels something, or the idea of being in charge of a process.
筦 combines 竹 (bamboo) with 完, which likely contributes the sound. The bamboo top suggests a bamboo-made object such as a pipe or key, while 完 may reinforce the idea of completeness or control.
Bamboo 竹 over 完: imagine a bamboo pipe that is perfectly complete and ready to channel water, or a bamboo key that fits completely into a lock.
For カン, picture a can of bamboo pipes: can -> カン, and the can holds the key to the whole system.