subwoofers loaded against dynamat?

Im wanted to side fire my sub under the helm for a little extra deep bass output, the problem is that the asthetic surround piece underneath the helm is a thin fiberglass/plastic material about 1/8" thick and wont be a good option firing against. The peice underneath is a full insert and I cannot be completely removed without a lot of work to make things look presentable.

Now Instead of struggling to reinforce that area, would it benefit from being covered with dynamat or similar material? I haven't had much experience with sound deadening so I'm not sure if it would work to keep the bass contained better or if it would just kill the bass response entirely?

Dynamat would not be good for creating rigidity if that is what you are after. That would require more access and a different material. However, Dynamat will effectively damp any panel resonance.

Yes I was trying to come up with an alternative to creating rigidity for sound waves to "bounce" off of. What would you suggest, maybe take out the insert and thicken up the fibergass with matting and reson?

You would absolutely benefit from having dynamat as the surface the sub will be firing against. It works wonders in absorbing the sound and keeping it in the area the sound is coming from. It would definitely help with that material rattling and help increase bass response.
So long as you're not trying to mount the sub to it and are just having it as the backing (think dynamat inside a car door behind the speaker) you will see benefit in that.
One thing I have done before to really help this in car audio is to put a few coats of rubberized spray on bed liner/undercoat on it, then apply dynamat to as large of surface area as you can that would be affected by the sound, possibly more so, then apply more rubberized spray on top of it.
When I used to compete in sound competitions, I raised my DB level a whole 5db just from sound deadening like crazy. There is definitely clear benefits to it, especially for stopping rattling and increasing bass response.

Hmmm, I would disagree with surf addict. Dynamat and the like are designed to absorb sound not reflect it. I believe you are more interested in reflecting the sound in your application. If you have a vibration in a car that is working against you then Dynamat will increase your output. In a boat I would suggest as reinforce the surface even if you simply add another layer of material (fiberglass, plastic, or a sealed wood product) right on top of what you have.

This is true. Dynamat is made to absorb sound and not reflect it. I also agree that doing a sealed box would be a much better option as well. If he has thin material that it is firing against, adding the dynamat prevent that sound from being wasted by flexing the fiberglass and rattling it will help increase the SPL in the enclosure and produce more bass in the cabin. That is what I was getting at.
Overall, if Ryan wanted the best sound, I would recommend doing a sealed box there, but that might not be an option. The dynamat can give him the results he is looking for without having to get down to fiberglass work.

Dynamat or any similar sound damping material can lower the noise floor and attenuate a number of types of unwanted reflections and resonance. But it cannot raise the overall output level, not by 5 dB or by any other substantial level. However, if you lower the noise floor you have effectively increased the dynamic range, which is definitely beneficial.
On the flip side, the added Dynamat will not attenuate the low bass output in any way since the wavelengths are so long. It would have to be some insane thickness to have that type of impact.
Go back to figuring out how to add rigidity to that thin panel.

This is true that just by backing an enclosure that a sub is firing on, that you will not get a 5db gain or anything like that. My 5db example I referenced was overall improvement at the dash in a sound competition and as I said, "like crazy". This wasn't just throwing a sheet of dynamat in the sub enclosure. It was putting spending days of time and over $1,000 retail in materials to do so. I put undercoating, dynamat, more undercoating throughout the car, on the floors, roof, doors, side panels, etc. In the cavities that could be, they were filled with expanding foam to make everything as solid as could be. The trunk lid itself more than doubled in weight due to all the sound deadening, the entire weight of the car gained around 100lbs from everything that was done for sound deadening. It is not a typical example by any means. I was just trying to illustrate the benefits it can have.