Unfortunately when the doublebass needs to play in large venues with big groups or with other mic’d instruments, the bass simply needs to have some kind of amplification. One of the devices I own for amplification is the under-bridge style piezo pickup. The make and model I currently use is The Realist sold through David Gage and designed by Ned Steinberger. Here is a picture of what it looks like:
Basically, the copper plate goes under the E-string foot of the bridge and two small piezo discs embedded in the plate convert the vibrations between the bridge and the top of the instrument into electrical signals. Then, those signals leave the pick up through the 1/4″ output jack (attached to the tailpiece) to a bass amp or pa system via 1/4″ patch cable. At that point the signal is converted again to a signal (which may or may not be processed through EQ or effects) that drives the speaker(s) through an amplifier.
Here is a picture performing with the band and choir of Bannockburn Baptist Church (Austin,TX) in a large auditorium that seats maybe 800-1,000 people.