Every bassist knows that one of the most important ingredients in their overall sound is the amplifier head. When you've got a powerful speaker cabinet to drive, you'll need a bass amp head that's up to the task. They come in a variety of sizes, power levels and feature sets, so with a little bit of browsing, you can be confident that you'll find the amp head that's right for your bass, your speakers, and your preferences. At its core, your amp head is a sort of translator. It takes the electrical signals coming from your bass and effects pedals and interprets them to create sound for the speakers to play. This means, of course, that the impact of the amp head on your tone is a huge one. Two separate components get the job done inside: the preamp and the power amp. Effects adjustments built into the head give you control over the function of these parts, which in turn allows you to customize your sound to your heart's content.
There are two main technologies that can be used for those preamp and power amp sections, and each one has its own distinct benefits. The original amplifier heads, made before today's modern electronics were invented, used vacuum tubes to control the sound. When the solid-state transistor was invented, it became another option to use instead of tubes, but musicians noticed something: they sounded different. That's why tubes are still so popular for amps—there's just no replacing that vintage warmth and distortion.
So, where does that leave solid-state tech? The takeaway is that they sound different; neither is better or worse than the other. You might choose a solid-state amp if you like melodic bass lines that demand precision and brightness, due to their highly technical output. Solid-state amps are also great choices for touring, since they tend to be really reliable and durable.
In the middle ground between tube and solid-state amp heads are the hybrid models. With these, you effectively get the best of both worlds. A hybrid model will usually have solid-state power circuitry, while the preamp section will feature vacuum tubes. This gives you the benefits of solid-state reliability and accuracy on the power side, and 'tubey' character on the preamp side, where the tone is first generated.
No two musicians are alike, so the right bass amp head for you is going to depend on your own tastes. Start by deciding whether you'd like a tube, solid-state or hybrid amp, then check out the effects, adjustments, and power levels of the amps you're interested in. From there, you'll be able to narrow your options down into a manageable list to choose from.