In most genres, a "clean" bass tone (without any amplifier-induced "overdrive" or "distortion") is desirable, and so while guitarists often prefer the more desirable distorted tones of tube-transistor amplifiers, bassists commonly use solid-state amplifier circuitry to achieve the necessary high output wattages with less weight than tubes (though smaller tubes can often still be found in the low-power "preamplifier" sections of the system, where they provide a warmer, smoother character to the bass tone for relatively little additional weight). A few all-tube bass amplifiers are still available, notably from the Ampeg brand.
Eight and twelve-string models are both built on the same "course string" concept found on twelve-string guitars, where sets of strings are spaced together in groups of two or three that are primarily played simultaneously. These instruments typically have one of the strings in each course tuned an octave above the 'standard' string, although a fifth above is also used. Instruments with ten and fifteen strings, grouped in five courses, also exist, as do "extended-range basses" or ERBs with non-coursed string counts rivaling those of coursed-string basses.
Maybe you've already got a specific brand in mind and want to see all the digital pianos they offer. All you have to do is click on the brand on the left hand side of the screen and within a few moments you'll have access to everything that's available. For that matter, you can always play is safe and stick to the top-rated items…and there are plenty of those in this selection.