As Bill says, if the cab is vibrating enough to throw the amp off, then you're wasting a lot of energy that should be going into creating your sound.
And while I know amps have to pass tests for various levels of vibrations, I treat my amps as delicate things. I wouldn't want any amp I own to be shaken around as much as this.
So you need a cab that doesn't waste your tone and volume in this way, and you need a vibration resistant foam between the cab and the amp.