I'm planning on using either the backless gang boxes or gang mud rings from Carlon. Since I'm doing DD in my theater room, how do I mount the gang boxes "out" far enough so that it sticks out far enough to attach the faceplate down the road? Are the gang boxes adjustuble for this type of thing or do I need to fur out behind where the gang box will mount?

I'm planning on using either the backless gang boxes or gang mud rings from Carlon. Since I'm doing DD in my theater room, how do I mount the gang boxes "out" far enough so that it sticks out far enough to attach the faceplate down the road? Are the gang boxes adjustuble for this type of thing or do I need to fur out behind where the gang box will mount?

you want to use these:

http://www.carlon.com/images/ContentPics/B121ADJ.jpg

You can adjust the depth of the box.

here is the cut sheet for the box:

http://www.carlonsales.com/productdrawings/B121ADJ.pdf

ace has them on-line, the big box stores have them too

http://www.acehardwaresuperstore.com/carlon-single-gang-pvc-adjujstable-box-p-17847.html?ref=42

I see, these have an adjustable bracket on the side. thx guys, I'll look for that type.

just a general rule of thumb here.. Be careful about the box sizes when using single gang boxes for your high voltage wires. This box is big, looks like 21 cubic inches though so you shouldnt have a problem if just running (2) 12/2 wires into the box.

From nec 314.16 you need 2.25 cu inches per wire plus 4.4 per outlet.
A real simple example is here
http://www.ci.richfield.mn.us/Police/Inspections/BuildingCode/electrical.pdf

This box will support (2) 12 guage wires.

They way I would do this in a multifamily where I cant use those plastic boxes(for high voltage) is use a 4" box with a 1 1/4" mudring - raco 775
http://www.hubbellcatalog.com/raco/RACO_datasheet.asp?PN=775&FAM=RacoBoxes