Blutack is a good idea. I was going to suggest museum wax or museum gel, it's similar product to blutack.
http://www.sculpt.com/catalog_98/finishing/Museum_Putty.htm
I haven't used this brand above, but it's all pretty similar.
Essentially it's reusable jelly or wax, often comes in something like a 35mm film canister, pull out a few small pieces, roll each into a ball and put at each corner of the speaker, press down to smash the wax balls thin. Velcro would work, but then there'd be a big gap underneath the speaker and the stand. I use museum wax to hold down several pieces of blown glass fine art that I have in my house. With museum wax, you can pull the stuck down object up easiest by twisting it free. You still could knock something over, but it'd take a lot more of an impact to do it. If you ever need a quick fix, use dental wax like for braces, and that's available everywhere and is cheap. It doesn't work quite as well as museum wax, but does pretty good for less money.