Acoustic amplifier

An acoustic amplifier handles acoustic instruments, where those for guitars may be the most common and bass as second. For a standard acoustic amplifier, any acoustic instruments can be used. Whether it be flutes, trumpets or percussion or anything else, these instruments have different tonal characters. Even more than a keyboard amplifier, acoustic amplifiers should be able to have a flat frequency response and needs to be distortion free. They require lots of feedback control.

This makes a difference, compared to guitar amplifiers where each of the manufacturer with various models have a certain sound for each model. A good acoustic amplifier, should have little of an own sound since it is meant to be used in a way that can provide a sound with a wide frequency response. Of course with various needs, there are different acoustic amplifiers too, especially when it comes to the level of power or ability to equalize the sound, if there are built in effects and so on.
Ordinary amplifiers feature too much midrange because the speakers don't provide a wide frequency response. A high-frequency tweeter is needed to reproduce high frequencies. Amplifiers with a low end have a speaker/woofer of a larger size. A common size is 8" up to 50W, 10" to 100W and 12" up to ca 125W. Some amps have 2 8" woofers. For a "hi-fi" approach, at least 10" should be used but only needed for instruments with a low range.
One of the pioneers in acoustic amplifiers was Trace Elliot, but today almost all manufacturers offer acoustic amplifiers.

What to be found in an acoustic amplifier

Just like guitar or bass amplifiers, acoustic amplifiers have tone controls and many models feature at least a three band tone control/equalizer. Within the tone settings, considerations may have to be taken to compensate for the acoustics in a live environment.
Furthermore, two or even thre channels, Volume, sometimes also input gain, effects loop, XLR input for microphones, a tuner output, a line out, footswitch control, extra inputs.
Some models are equipped with digital multi effects like delay, chorus, compression and reverb. There are also other rotary effects as of vibrato or a leslie.

Acoustic audio feedback

Due to the risk of microphone feedback, some amps also provide feedback suppressing technology, with a parametric equalizer, as a notch feedback filter. Some better amps, such as of Crate, have an auto feedback elimination circuit. Acoustic pre-amps (without final amps), commonly have a simple phase switch that may minimize feedback somewhat.

Distortion free amplification

In order to provide the needed headroom for distortion free reproduction, higher power may be valued since the high frequency tweeters would sound very bad if any distortion would occur. A compressor/limiter would prevent distortion, but would also affect the sound. So the best would be enough power and a distortion-preventing limiter.

Acoustic guitar amplifier

An acoustic guitar has a wide frequency spectrum. Acoustic guitarists need amps that's more related to a hi-fi system than a Marshall stack. Some models, as the Roland AC60, only use 6.5" speakers which may be too little.
Some acoustic guitar players value an acoustic bass amplifier to get the lower end, that otherwise may be only provided in the top acoustic guitar amp models that are better in more ways but also more expensive.

The number of tone controls should be at least three. Better models are equipped with more bands as well as one or more parametric eq.

Acoustic bass amplifier

Pretty much quite alike amps for acoustic guitars but demanding the low end. Even though bass notes don't go as high on a guitar, the sound of touching the strings while playing, may be part of the whole sound. Therefore, as other acoustic amps, a full frequency response may be required, achieved with a hi-fi tweeter.
Some acoustic bass amps have a pair of 8" speakers even up to 200W but it can be questioned how well the lowest frequency responce may be with that size, even though a rear driver is used. A 10 or 12" speaker would however ensure a lower frequency response.

Link:
Aspects about acoustic feedback - www.acousticpreamps.com