Two ways casters harm sound quality. Main one is vibration control. Whatever the speakers are on has a huge effect on sound quality. Casters are rubber wheels on bearings, with some rake angle thrown in for good measure. None of this is good. All guaranteed to smear transients and lose fine detail.
Second way is casters swivel around, making accurate speaker placement a lot harder, and leveling all but impossible. The speakers will never sound as good simply because they can never be in exactly the right place.
I get why someone who has to move big heavy speakers around would be willing to put up with all of that, it is nothing compared to the hernia, ruptured disk or good old back pain you can get wrestling big speakers around. But you ask about sound quality, you get the sound quality answer: Put the casters on a platform, put Podiums on the platform, put the speakers on the Podiums.
Second way is casters swivel around, making accurate speaker placement a lot harder, and leveling all but impossible. The speakers will never sound as good simply because they can never be in exactly the right place.
I get why someone who has to move big heavy speakers around would be willing to put up with all of that, it is nothing compared to the hernia, ruptured disk or good old back pain you can get wrestling big speakers around. But you ask about sound quality, you get the sound quality answer: Put the casters on a platform, put Podiums on the platform, put the speakers on the Podiums.