Originally Posted by
GreenTennis 
When I received my DT ProCinema 800 package I discovered the subwoofer had a bad case of the rattles. While trying to isolate the issue it was discovered that the rattles would disappear it the unit was placed upside down. I then found this was not the first report of this issue. I found the included thread that discusses this very issue. Below is a solution for those that have the same symptoms. Hope this helps those who have this issue.
http://www.avsforum.com/avs-vb/showthread.php?t=1271997
When I found this post I discovered this is not an isolated case of DT ProSub has rattles. Case rattles that do not occur if the unit is turned upside down. If you have this issue and your sub is under warranty or you purchased it local you might want to return it for another. However for those that would like to try and resolve the issue then see follow the guide listed below.
Symptoms:
When power is off to the unit and you tap on the side of the unit you hear a rattle along with the vibration. To test for the rattle, perform the following steps:
1) Disconnect all the wires to the sub including the power cord
2) Tighten the speaker connectors if they are lose
3) Place the sub on carpet or soft surface
4) With a closed fist, tap on top rear corner of the sub with the soft part of your fist. Listen for the rattle or buzz.
5) With your other hand slightly lift the rear of the sub and repeat step 4. The rattle/buzz will follow the vibration you feel. If the rattle is not present, it is possible you have a different issue.
6) If you hear the rattle, gently place the sub upside down and repeat step 4. If the rattle is all but gone, then there is a good chance the steps below will show and resolve the rattle issue.
Disclaimer: Proceed from here at your own risk. Care must be taken to keep from damaging the unit.
What you will need to perform this task:
1) A Philips screwdriver (medium size)
2) 1.5 to 1.75 thick book
3) A wrist sweatband
4) Flashlight
 
The Procedure:
1) Place the sub right soft surface or with a cloth underneeth
2) Remove the front grill and store out of the way in a safe place
 
3) Carefully pick up the sub and place it face down evenly on the grill holding pins.
4) To remove the screws, do so in a staggered pattern similar to removing and installing lug nuts on a car. If you don't want to mar the screws or back plate, place a piece of cloth between the screwdriver and the screw. Remove the screws slowly to minimize the loss of particle board. Don't be surprised if some screws seem tighter than others. That is the result of them using wood screws without T-nuts.
When removing what I thought would be T bolt/nuts, I discovered they were actually wood screws into particle board. This can create problems and is not a good practice. I was very disappointed to find out they designed and manufactured this unit this way. You will need to be very careful when removing the screw and even more so when screwing them back in to keep from striping the wood. The problem with wood screws into particle board is that the glued compress wood will crumble away from the threads causing them not to bite into the wood. Even with care this can happen. If it does, then a trip to HD/Lowes to pick up some T bolt/nuts and do the job correctly.
5) Very carefully pick the sub straight up keeping your hand on the back plate to make sure it does not fall out. Being careful not to put any side stress on the front grill pins while lifting, rotate the sub back to the upright position keeping pressure on the back plate. Sit the sub down with so the book is right at the edge of the back of the sub.
6) There is very little slack in the wires so be careful when sliding out the plate. There is a good chance the plate will be stuck to the speaker box, so you might have to work a little to get it lose. Do not pull hard on the speaker knobs or use a screwdriver to pry it loose unless you do not mind cosmetic damage. If you are having a hard time getting it loose, try taping the box or lifting the back side ½ and taping it down. Make sure you keep your hand on the plate while you do this. Once it is free, slide it out slightly. Stop if the wires are tight and stressed. The wires are tied to the top speaker box with a wire tie. If you need more slack, undo the wire tie. You might be able to do the repair without undoing the wire tie if you can get the back plate out enough to sit on the book.
7) At this point you will notice there is block of wood glued to the base of the sub box that sits just beneath the transformer.
Not sure if the manufacture forgot to put a pad on top of that wood block or if they intended for it to be like that. Assuming the purpose of that block of wood is in some way there for support of the transformer, it just does not work as is. If you look at the transformer you will notice that is it mounted on an L bracket. Do to the weight of the transformer, it flexes. I could not tell for sure if the transformer flexes enough to hit the wood. I would have suspected evidence of rubbing and found none. However, just the vibration of the transformer would be enough to cause issues. It was nice to see that they used Loctite and sealant on the components and connectors.
The mystery block of wood and the focus of the modification. Also notice the wood screws used to mount the passive driver and the main driver.
8) Take the wrist sweatband and place it on top of the block of wood. Tuck the end down into the space between back cover and the block of wood. This will help in keeping the sweat band from sliding back when the back plate is re-inserted.
9) Retie the wire tie that holds the wires if it was undone and slide the back plate back into the enclosure.
10) Holding the back plate in place with palm lift up the sub and place it face down evenly on the grill holding pins.
11) Install the screws in a staggered pattern similar to removing and installing lug nuts on a car.
Do not over tighten as this might strip the particle board. Once all the screws are installed, gently go back over the screws until they are tight.
12) Pick up the sub straight up being careful not to put any side stress on the front grill pins while lifting, rotate the sub back to the upright position.
13) Test for the rattle as described in the systems section. If rattle still occurring, check the screws to make sure they are tight and that the speaker connectors are tightened. Repeat process if necessary.
14) Install front grill cover.
I hope this helps those who have this problem with their DT ProCinema subwoofers.
For me this eliminated 96% of the rattling. Only rattles that occur now are when the subwoofer is pushed at a very high volume. With a little more work on the inside of the cabinet and using T-nuts, I bet this cabinet could be 100% rattle free.
For Fun:
If you would like to hear the transformer hum, with the volume all the way down put your ear on the top of the subwoofer cabinet.
In all seriousness though, you said that you place the calibration mic in the middle of the room. Is that your main listening spot?
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