If it is not prohibited by the Constitution then there is no reason why it is not permissible. It's just a question of how you go about it. Unilateral secession as practiced by the Southern states wasn't the way.
> If it [state secession] is not prohibited by the Constitution then there is no reason why it is not permissible. <
That is a very good argument, especially if the 10th Amendment is considered as well. But as I noted earlier, it would have been nice had the Founders removed all doubt.
By the way, is anyone aware of anything the Founders might have written on this issue? Anything in their private papers, etc.?