Are the legs on Ikea Lack tables solid ? If not what do the spikes go into ?

Nice speaker stands BTW. They look well proportioned for your speakers. When i was using Atacama SE24 stands a few years ago (a broadly similar design to yours),  i found that if I put too much sand into the stands it would suck the life out of the speakers, so i ended up leving a little air gap at the top to kinda let them breathe a little.