In a lot of gigs I've played, I have noticed that they have placed the amplifier speaker cab (usually just the bass amp) on top of a chair or a stool or something, so as not to be on the ground.
What is the benefit of something like that?
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It is so the speaker is pointed more towards your head than your feet and so you can hear yourself better. If a small combo-amp is on the ground, the sound has to bounce quite a bit to actually get to your ears and if you are at a band practice or a gig it may mean bandmates can hear you better than you can hear yourself. If your speaker is pointed at your head you can usually cut down on a lot of volume and still hear yourself just as good then if it was louder and pointed away from your head.
Another trick some people use is they tilt the combo-amp back so they can leave it on the ground and still have the sound from the amp directly pointed at their head. If everyone in the band with an combo-amp does one of these tricks it can seriously reduce stage volume.
In all my experience as a sound technician the number one issue is and will always be stage volume being to loud. It leads to many, many problems including:
Any one of these is a serious issue and have to be dealt with for live sound. The bass being on the low end of the frequency range has to be kept even more in check because of feedback and issues with Fundamental Frequencies. The less intense (loud) the sound is the less problems it causes. Also bassist will turn up if they can't hear themselves and every speaker I've ever seen is louder if you point it at your ears because that is how they are designed to work.
I see that you noticed this happens usually with the bass amp, and I don't see an answer addressing bass in particular.
I can think of some reasons:
Bass amps are isolated from the floor to avoid mechanical coupling.
Depending on the venue (stage design, materials, acoustics, etc) floor vibration can cause an array of problems, like triggering the snare and inducing movement noise on mics. This can be specially bad and nasty if a particularly strong resonance occurs, not only for the noise it will cause, but for the added potential of feedback.
Another possibility for that specific amp placement is sound-design. If microphones are being used, the timbre of the instrument and amp combo will be different depending on the placement of both the mic and the amp.
"I see another answer explains that the positioning is to "hear yourself better", but this will be hardly the case for bass amps, since those wavelengths are not very directional. Bass amp isolation from the floor will make little to no difference; low frequencies, the ones produced by the bass, are omnidirectional (unless a special system is used, which is not the case of bass amps)."
Bottom line is you don't have ears in your knees, so you need to point the amp at your head a little bit so you can catch some of the sound being played.
Worst case of NOT doing this I've experiences was a keyboard player who put his amp under the keyboard (in front of knees), pointing forward. He could hardly hear himself, but to the audience he was blisteringly loud. Turned it up more to hear himself .. it didn't end well.