The only way that you will truly know, is by experimentation, just like most of the rest of us. There are guys who will build 10 boxes until they find the right one. Just experiment with it. build the big box, play with filling it. Hell, if you dont like the sound of it sealed, it will be big enough that you can toss a port in it. You might even learn something along the way, and improve your box building skills. If you have a shop build it, you will be screwing yourself, because you will only have the one enclosure built prolly @ 4 ish cubes, and will not know what 6 or 8 sounds like. Not to mention that youll pay an assload of money. They see you coming with that RE, they'll think your made of money. Same thing when a corvette owner rolls into a mechanic shop.
Good luck