Boomy is a result of a larger compresion chamber or a small sealed chamber.  Try reduceing the volume of the compression chamber- if it doesn't work, its the Sealed section.

I don't see why turning the driver around would be a problem. this would serve a dual purpose.
I would also try some sandbags to the CC.
if it works you may want to move the driver closer into the CC if your changes work.
the things heavy enough why keep a sand bag in it.

PS A 12" WO is always cool

[quote author=bnew63  link=1170798603/0#4 date=1170817527]Gex
The sand bags would only be temporary to calculate how much I need to decrease the CC.
I cheaped out and made this one from MDF so it's heavy.
Getting the 12" drivers to fit was a squeeze so I'm hoping I won't have to move them

  Brian [/quote]

Ya I know, It was a rather stupid thing to say on my part.
Did you ever calculate the sealed area for that sub before you built? or just build it to Steves specs except for the height of course.
I see why you are reluctant to move them.
are you going to flip them around?
I had used insulation foam to take up space once. lined the CC with blue stryo. Even with styro/wood glue I felt that it may rattle loose after time.
Vibration and the glue hardens and becomes brittle and it falls off. It was  a Temp thing only.

Good Idea
silicon will stay flexible. It can be a bad word around here although It never been an issue with me.
"The dreaded foam surround decay syndrome"

When selecting woofers for this enclosure, you can model a regular 4th order single reflex box to determine the rear (sealed) chamber. If it wants less air space than you have already built (gee you made the box first?) simple add bracing to the sealed chamber until its internal volume matches the requirement of your woofer. Do not adjust the baffle further back in the chamber to reduce its size as a short cut, or you will increase the Q of the box to well over 1.0 (boom boom)