First off, that is secret 5, not 6, the secret has been known about for quite some time, the cube gives you another way to get into the HUB room, just by simply pressing F5.
The order of secrets are well, stage 1-5, unless you were counting a secret boss, i really have no idea, secret bosses are their own thing.
It's probably best if you ask around about this secret before posting it on the forum, because i'm going to be honest, everybody knows this exists.
I think that is all that needs to be said here, i'm going to lock the topic, since, what else is there to say, any other moderator or admin feel free to unlock it at your own will if you have input.