Proper speaker choice

I have purchased an MTX thunderform for my dodge ram pickup. This box came preloaded with a amp and to mtx 10" subs. It sucked. I bypassed the built in amp, removed the 2 mtx subs and added 2 RF punch subs with a puch 300 amp, I have the amp bridged to one channel as well. I am still not pleased with the ammount of base that this produces. I would figure that 2 10" subs would "slam" inside of a pickup truck. I seem to hear the speakers bottoming out on certain base tones. I guess my question is, Seeing the box is a molded plastic box thats holds 2 10" subs and is mounted under the rear bench seat that there isent that much "air space" in the box. I would like to know from the professionals from the field, on what speaker that they would reccomend for this size enclosure. I would really like to keep the amp because I know it has potential and it is just right in size. Now I am not looking for the chapest speaker out there but I also dont want to break the bank either. I am runnig a alpine CVA-1003 head unit, and still have the 6 speaker factory infinity speakers which seem to sounfd fine with the head unit.

Thanks for your help and ideas

• Location: Under rear seat
• No loss of leg room
• Bolts to vehicle with supplied hardware (cut out in rear of ThunderForm allows factory jack arm to remain in current location). Jack is unaffected.
• Absolutely seamless-no seams to fail
• Molded in factory colors
• Subwoofers: Two 10"
• Install Sheet in PDF format. (751kb)

Dimensions: 8.687" x 48" x 17.5" Weight: 38 lbs.* Power Handling: 450 Watts RMS* Sensitivity: 87.4dB* Enclosure Volume: 1.7 cubic feet Impedance: 4 Ohm*

Now, I couldent find my model amp because it is last years model, But here is this years model. I beleave that the output is the same.

http://www.rockfordfosgate.com/products/ez2rockford/ModelInfo/ModelInfo.asp?ItemKey=10063520&CategoryDesc=Punch%20Amplifiers

My bet is that you're clipping the signal somewhere if you hear an audible, "bottoming out" sound.  This simply shouldn't happen with sealed enclosures.  Honestly, you might just have unrealistic SPL expectations from the amount of power and subs you have down there.  Even if the gain is set correctly, you might jack up the volume to attempt to meet these expectations, and clip the signal. 

I highly doubt it is a problem with the particular subs, especially since you've tried two different pair.  It would probably be a waste of time to buy another set. 

If you really want significantly higher SPL you'll probably need to use another application (box) and a lot more power.  This probably isn't what you want to hear, but it is my opinion based on what you've told us.

So,

Do you think that this could be the cause of the speakers both bottoming out and not "hard hitting". I think I should stick with two 4 ohm speakers.