Speaker stands are considered critical by many in the overall sound waves that arrive at your ears. Manufacturers go to great lengths to ensure that the bookshelf speaker cabinet is, among other things, rigid and with the least resonance to color the sound coming out. It would be ideal if you could suspend a bookshelf speaker in mid-air rigidly. However, gravity requires that you must put them on top of something, whether a floor, a shelf or a stand. In your case, plastic monobloc chairs.
Unfortuantely, most surfaces on which a speaker rests have a nasty way of sympathetically singing with the speakers when they start to make sounds. That's called resonance. So you hear the furniture together with the speakers. I am not sure of the resonance of plastic monobloc chairs. Perhaps plastic isn't as bad as hallow wood or metal when it comes to resonating. I haven't tried it myself. But if rigidity and firmness are a big deal among speaker makers, I will have to think that plastic, however solid, would not figure in their list of speaker stands.
Sand or lead filling is a common practice to dull the resonance of most commerically available metal and wood stands that are hallow. Making them as heavy and as solid as possible often pushed their resonance outside of the audible range. Moreover, the added weight helps preclude any chance that the stands might move together with the speakers during loud sounds. In any speaker, whether floorstander or stand-mount, only the driver cones should move, nothing else.
So, if you don't listen at loud volumes and you don't find anything objectionable to the sound waves that reach your ears from where the speakers are, I see no problem. It's often the mid-bass that suffers, getting muddled, when using non-rigid stand-mounts. And in the case of hallow metal stands that ring, you could modulate the mids and highs as well. But I think if you listen quite close to the speakers(called nearfield listening), most objections on speaker-stand combinations can sometimes be dismissed. Just my thoughts.
