To sound like a regular double bass you need a Double Tub.

Well, actually you don't; but that one's served quite well for over 15 years, and hardly ever been in out of the rain.

To get more volume, you really need a much heavier string, and a heavier string means you need much more tension. The one at the link runs at around 60 pounds, which is lower than seems common on regular basses, but the sound is close to a 3/4 concert bass - the common size used in bluegrass bands.

John