The first thing that you should consider is how and where your amp is going to be used. For example, is the amp for home use or for playing in larger venues such as halls, pubs, clubs, etc?
For the beginner or home use, the bass amp is likely to be a solid-state model that does not have any valves in it. These amps are also sometimes referred to as ‘combo amps’, as the speaker and the amplifier are built within the same enclosure. Most brands in this category offer power outputs between 15 to 100 Watts and the power rating is usually mirrored in the physical size of the speaker, i.e. the larger the speaker, the more powerful the amp is! Common combo speaker sizes include 8 inches, 10 inches, 12 inches and occasionally 15 inches. The size of the speaker also has an influence on the amplifier tone, with larger speakers giving a fuller and warmer sound.
How much power??An amplifier that appears to be really loud in your home is likely to be very underpowered if you try to use it in a live band situation. Sound and ‘loudness’ is all about moving air and as you can imagine, your living room has a lot less air to move than a hall or a pub. If you and your little combo amp are trying to compete in a live band situation with a loud drummer or perhaps a guitarist with a valve amplifier, then you will definitely find yourself fighting a losing battle!
When considering playing in pubs, halls, clubs, etc, it is really important to pinpoint the exact bass sound that you are trying to achieve before you buy an amp. For example, are you looking for a tight sound with punch? Or are you trying to get a big, full bass sound with lots of low-end tone? Do you go for an all-in-one combo-type amp, or should you get a popular bass cab and amplifier head unit? If you go for a cab and head mix, the cabinet can either house a single 15 inch speaker, two 10 inch speakers, four 10 inch speakers, two 12 inch speakers, or a single 12 inch speaker and so you will also need to decide which suits you best.
The amp head can be anything from 100 Watts up to 1000 Watts. Consideration needs to be paid to the power type of your head, as this will help you to decide whether you go for solid state heads (which tend to be a cheaper option) or all-valve amplifier heads. All-valve heads usually offer 100 to 200 Watts of valve output. With solid-state amplifier heads (which sometimes include a pre-amp valve to colour the tone), the output can be higher, at 300, 500, or even 1000 Watts.
The next step is to match the amp output with the cabinet handling. For example, it would be risky to have a cabinet that can handle a maximum of 300 Watts, matched with an amplifier head with a power rating of 500 or 1000 Watts, as you could potentially overdrive (and consequently damage) the speaker(s). A 100W all-valve amplifier will easily compete against a solid-state head rated at 250 to 500 Watts of output power. If you decide to go down the combo route, you will typically find that the power output is less than what you will find with amplifier-cabinet combinations.
Leading valve amplifier brands worth considering include Orange, EBS and Ampeg. Companies such as TC, Mark Bass, EBS and Line 6 all produce excellent amplifier heads. If you are after a combo amp, then Line 6, Mark Bass, TC, Orange, Ampeg and EBS are all worth looking at.
At the end of the day, it is down to you to consider the sound that you are looking for, but as general guidelines, 10 inch speakers give you tight sound with punch, 15 inch speakers give a big, full, fat bass sound, and 12 inch speakers are somewhere in the middle, i.e. still fairly punchy, but with a bit more low end than the 10 inch speakers due to their larger size.