Re: Casters, are they tone robbers?
This question is not quite as tone-geeky as "Does the brand of battery I use in my pedals change my tone?" but it's getting close.
If it's a big concern with you, you should know that Auralex even makes a product, a foam pad that you set you amp or speaker cab on, that isolates your amp from the floor to prevent vibrations from being transmitted from your speaker cab through the floor. Get one, and you will hear only the pure tone of your amp, unaffected by its environment. Oh wait, that's not quite true. You must deaden all reverberation off the walls of the room you have your amp in, since those also change the tone that ends up reaching your ears.
My.02: In a studio, you often just put a mic up to the grille cloth of your speaker, so the mic isn't gonna pick up much of what the floor and rest of the room is doing to the tone of your amp. In your living room or bedroom, enjoy the whole tone experience - the amp, the walls, the floor, the furniture... just play!