Bass Guitars generally put out a very low voltage signal at the output jack (usually in the micro-volt to milli-volt range). Hence, the need to boost the signal via an amplifier or amplifiers.
Bass amplifiers come in variety of configurations: Combo Amps, Pre-amp and power amp combination, and the generic bass amp head unit, which has both pre-amp and power amp in the unit. They also come in tube type, solid state, hybrid and digital.
Combo amps have pre-amps, power-amps and the speaker all in one unit. Below is photo of a Behringer Bass combo amp. This is a 120 watt unit with a 12 inch Buruga Aluminum cone woofer with a rear vented box.
Below is a photo of a solid state (all transistor) bass amplifier. This is a Peavey Delta Bass Amp. This series of Peavey amps (Delta, Session, Nitro) all have the same front end (pre-amp) circuitry.
Bass amplifiers are full spectrum amps, which usually require a lot of power(measured in watts). This is because lower frequencies require more energy to develop. The lower the frequency, the more power that will be required to get the same volume. Hence, if you switch from a four string bass to five string bass (where the low string is a B) and you wish to the same volume for the B string as you did for the E string, you will need more power from your system. Note that I said system, and not amplifier. This is because you can easily get more volume by using a more efficient speaker system. Sometimes this can be accomplished by porting the cabinet, which utilizes the radiation from the back side of the speaker and correctly phases this with the front side. Often you get double the equivalent of power by doing this, but you have to be precise in your calculations and construction of the cabinet.
Electronic amplification for the bass comes in three basic types: All Tube, All Solid-State, Hybrid (Tube and Solid-State Mixed). From an engineering stand point, the solid state has a flat equalization effect on the bass, or no EQ. This is the nature of transistor amplifiers (also known as solid state).
However, most musicians prefer the sound of tube amplifiers. This is because tubes color the sound, and they color it nicely. Generally, there is nice high frequency boost which occurs in each gain stage inside the amp itself. However, there are some down sides to tubes. For starters, they are not cheap. Certain tubes can be hard to find. There are not a lot of companies making them any more. Tubes generate lots of heat too. They require heavy transformers for the output stage and power supply. Also, you are going to be limited to 300-400W maximum for this type of system. An all tube Ampeg SVT amp-head weighs in at 82 pounds. Tubes also need to warm up for about 20 seconds or so, before being ready to do their job. Tube amps must have a load connected (a speaker system connected to the amp) or the tubes will burn out and you will blow the output transformer quickly, which is not cheap either.
Hybrid amps, usually consist of tube pre-amp sections and a solid state output stage or stages. You get some of the warmth of the tube sound and the power of solid state (1KW, 2KW, 3KW …) with out the weight, heat, and extra cost.
It is now fashionable for amplifier designers to make digitally based bass amplifiers. Below is the Peavey DPB, which is a 300 watt, digital amplifier system. The controls are very straight forward, which is really nice. This amp has a very clean and warm sound. The cabinet below it is a McCaulley 15 inch ported cabinet.