Rhys, the main function of a preamp is to boost the pickup signal sufficiently for it to drive a power amplifier, but a preamp is also used to power electronics that are used to color the signal (eg. EQ) before it is amplified and sent to a speaker. Combo amps have an inbuilt preamp circuit as well as a power amplifier, while rack rigs generally use a standalone preamp and a separate power amp. Musicman ("active") basses have preamps as well, to provide the player the ability to dabble with their tone more than you can with a "passive" (no preamp) bass.
Tube preamps are mainly used for recording vocals and instruments, to remove the harsh, "dead" sound that solid state or digital equipment gives to sound. Tubes introduce a "warm" natural sounding tone that is more pleasant. For this reason, tube preamps are used extensively by bass players, live and in the studio, to obtain a nicer sounding tone than is often the case with solid state preamps.
Since you're in Australia, I can tell you from first hand experience that tube preamps for bass are not commonly available, other than in high end and specialist stores, and for this reason you are almost always better off buying from overseas. For example, the Alembic distributor in Australia sells the F1-X tube preamp (renowned for the past 20 years as probably the best bass preamp ever made) sells them for AUD$1400 (that's around US$1000 for those of you playing at home), whereas the same unit retails for a street price of around US$650 at large US bass retailers. You can get the same preamp, or the comparable Demeter VTBP-201 (which I have) for around US$400-$500.
However, there are cheaper options if you want to buy a tube preamp and see how it sounds. Behringer make a tube preamp that is quite suitable for either mics or instruments, including bass, that retails for around AUD$250 (US$175) downunder.
There are also a number of other tube preamps you might be able to find on the second hand market in Australia, manufactured by Ampeg, Eden, Ashdown or SWR.
