Q: I’ve seen a lot of engineers recommending putting a guitar amp on a chair or stand to get it off the floor. Why is this?
A: In most cases, it has to do with acoustics: when a sound source is adjacent to a reflective surface (in this case, the floor), the low end will be boosted by 3dB. In some cases, you can also have problems with sound reflecting off the floor and getting into the microphone causing cancellation issues.
So, for uncolored bass response from the amp, most engineers recommend that it is best to raise the unit off the floor.