At the very least I guess you can look at this as a learning opportunity. You are right, if the whole speaker cabinet is moving that is dissipating energy you want coming out of the driver. So its robbing you of dynamics. Also if the speaker wasn't so loud you could hear the sound coming from the cabinet itself, which is smearing and robbing you of detail. This is why all the very best speakers are so thick, solid, and massive. Ideally you want no vibration anywhere at all except from the driver.
You can try several blobs of Blue Tack to hold the speakers in place. Or BDR Cones which are another better form of vibration control. Either way yes lead shot in a bag will add mass. Lead shot in a bag however is more damping than controlling. Only way to know is to try and hear how it sounds. This is why all these tweaks should be considered learning. Which is a great thing to do in its own right, especially if you keep it cheap, simple, and reversible.
You can try several blobs of Blue Tack to hold the speakers in place. Or BDR Cones which are another better form of vibration control. Either way yes lead shot in a bag will add mass. Lead shot in a bag however is more damping than controlling. Only way to know is to try and hear how it sounds. This is why all these tweaks should be considered learning. Which is a great thing to do in its own right, especially if you keep it cheap, simple, and reversible.