The double bass has the exact same range as the bass guitar but has a much longer scale length--usually about 41 or 42 inches although it can be even longer or shorter depending on whether you're playing a 4/4, 7/8, 3/4, 5/8, 1/4, 1/8, 1/10, etc. 3/4 is a kind of standard. No one uses 4/4 anymore and I wouldn't use a 7/8 either--they're just too big and it's hard to get up and down the fingerboard on those

Because the scale length is longer, it's harder to finger which is why we are taught to use the 3rd and 4th finger in unison (in the old days, it was the 1st finger and the other three in unison). You can transfer that over to the bass guitar, though, and it works all the way up the neck. You don't need to adapt the double bass thumb positions or anything. When I said your 1st finger should always be resting on a string, that was for fingering--not hitting a string open, of course--just wanted to clarify that.

As far as amps go--the best double bass amp is the SWR natural blonde. By far, the best on the market. Small but POWERFUL! A full 200 watts of power and it amplifies the double bass sound rather than sounding like a bass guitar. That also depends on the pickup. The best pickup is the Yamahiko out of Japan--whose founder invented the double bass pickup. A cheaper model that also works very well is the Underwood.

Then there's strings. Never use the factory-installed strings. Always get a new set. I use D'Addario Helicore medium tension hybrids. My instructor liked them so much he ordered the light tension set which are also very good. I would recommend steel strings at any rate but there are people who swear by gut strings like Velvet Anima or something. I recommend steels.

You can get any double bass accessories from Lemur Music online. Don't have a URL but just type Lemur Music in your browser and you'll find it easy.