I found something that worked for me (standard skullworx kit in an MRCE helmet)!
The ultimate fix:
1) remove the foam from the boom mic.
2)Place a piece of tape over the back part of the mic (the part facing out towards the helmet, opposite from the part you actually speak into).
3) Replace foam
I found that the feedback was being caused by the omni-directional mic. The boom mic actually has 2 microphones (if you take the foam off, 1 on the front and one on the back): one by your mouth, and one on the back of this. Because of the confined space, the mic was actually sitting too close to the plastic of the helmet, which was createing the feedback.
After taping the mic facing the helmet (essentially creating a uni-directional microphone) I tested with the skullworx amp and was able to turn it up just about all the way without feedback!
I did some slight modifications and replaced the skullworx amp with a TCI. Again, I was able to bring it up to almost full volume with little to no feedback!