Should there be something between a speaker and the surface it's on, something to cushion it? I've always used foam cushions but a friend of mine told me that was problematic. Is that true?
I had not put anything beneath my speakers ever since I owned one. (I put mine on a shelf for all my audio-related tools made of wood.) I don't think I've ever used them with any form of cushion. Is the sound different? What did your friend tell you?
It depends how big the speaker is and how loud you turn the volume up. I would go for the foam if your speaker is moving when its on and a moving speaker is usually the case when you turn the volume so loud.
We can use a double-sided tape so that the speaker won't move from the surface. I think the small space created by the tape is good enough to cushion the speaker when the volume is high.
Most speakers have a small rubber underneath it. If your speaker doesn't have it, then you can ask one at your local electronics store and they usually have those.