Unscrew one of the spikes and take it to a big box hardware store. Go to the fastener section and gently try the spike's threads in the thread sizer until it goes in smoothly. Record the size and search for a caster with matching threads. Verify the load rating of the four casters to support the weight of your speaker.

No drilling. No foolish sliders. 

I fitted commercial casters to my 400 lb + per speakers to position them and after marking the floor with tape, I left the casters on in case I ever wanted to change position.  I never have and I doubt that being on casters rather than spikes affects the sound.  

I wouldn’t put casters on speakers any more than I’d  put spikes on a moving crate especially when there are products like Herbies Gliders built for the purpose, but that’s me.  

Try threaded stem mount leveling casters from Footmaster. These will let you roll the speakers around on casters then drop them down on rubber pads built into the caster foot and level your speakers at the same time. What you have to do is match up the threads your speaker uses for its spikes to the casters. If they are not the same you can get threaded rod that changes where one end fits the caster and the other fits the speaker.

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