Anyway, to respond to your questions:

>> 1) It seems pretty solid right now, do I need more bracing?
>> For example do I need to put anything on the top bottom panel centers?

From what you've said, I think you've used adequate bracing. It generally can't hurt to add more though, unless you add so much material that you appreciably reduce box volume in doing so. A couple hundred cubic inches displacement from bracing in a cabinet the size of Theater four π speaker cabinet is not a problem at all. So you can add a couple more braces if you'd like, but I think you've done just fine.

>> 2) Time to make it air tight. What do you guys recommend to seal
>> the insides of the mdf panel corners? How about pl-400 or one of
>> those window sealers? What have you guys used successfully?

I think maybe the cabinetmakers should address this, but hopefully your joints are airtight already. Usually the seams are tight and glue together firmly. I know that bisquit joints sometimes tend to add some space, but I'd really prefer to see the joint be snug.

If there are gaps that cannot be filled with glue, I'd probably suggest silicon seal.

>> 3) What do you recommend I seal all the removeable parts with?

The woofer is gasketed, and I usually don't add any gasket compound to it. if you do, then you will damage the gasket if you ever remove the woofer. Usually, if the hole is uniformly round and the mounting surface is smooth, the dry gasket seal is airtight.

The tweeter is sometimes more of a problem. Generally, if it is centered on the hole, it will seal too. But you should test for leaks after assembly. When playing some material with a lot of bass fairly loudly, feel for air leaks with your hand. Run your hand around the edge of the tweeter, the amplifier connection panel and all edges that you are concerned about. Leaks will be prety obvious as gusts of air, if you are playing content loudly and with a lot of bass.

If you find leaks, you can fill the gap with silicon. The tweter might be removed and a bead of silicon gasket material run along the inner edge of the horn mouth. You may not need silicon sealant there and I don't think you'll need it anywhere else. But if you find leaks, that's a good way to seal them.

>> 4) What is a good way to mount all the electronics in the box,
>> worried about vibrations...

I like to bolt the large woofer Zobel resistor and the crossover PCB on the bottom interior, others prefer the back. Use a wood screw large enough to bite into the wood but not so long that it goes through and pokes out. Then lay the insulation over the Zobel resistor and the crossover PCB and cut a slit for the wires to pass through.

You can tack the Zobel capacitor down with glue or silicon if you like. You can also tack down the tweeter cabinet assembly onto the crossover in the same way. If you use a glue, be sure it is of a type that is semi-permanent. The silicon compount works good here too; Another thing that works well is weatherstrip adhesive. You can get that at any car parts store.

But you can also just lay these components on the R13. It is thick enough that it will prevent the Zobel capacitor from vibrating against wood. The block of components on the tweeter cable assembly is pretty rigid and doesn't have freedom to move far enough to run against the wood, but it could vibrate against the PCB. But with the insulation separating it from the crossover board, it will be padded just like the Zobel capacitor and won't vibrate.

>> 5) What is the best way to attach the r13 to the mdf.

Some people tack it in with a staple glue. Four or five shots will do. It's fast and works well, that's how I've done a lot of speakers. But if the staple gun isn't strong, somtimes the staples aren't driven in well and they can come out. I donb't like the idea of debris inside speaker cabinets, especially pointed metal debris. So I prefer to use white glue. It takes longer and is a little bit messy. But it ensures a good bond that's free of debris. IN either case, staples or glue, remove the paper water barrier from the insulation before installing it.

Glad to hear your making progress. Please keep us posted, and post some photos here if you can!