i don't think it will be louder, there'll probably be difference in the sq between two subs though. and i've read here and been told before that it could actually harm the sub by underpowering it. i would just wait until i could afford an amp to run the new sub off of before putting the sub in.
underpowering subs doesn't hurt them, but in doing so most ppl want to crank the volume to get the sub louder, in doing this the amp clips, sending a square wave to the sub, which is bad, this causes distortion which over time will ruin the sub, i would just keep the d3 untill you can upgrade the amp.
Ya the D9 12 is 800 watts rms but this is a 10inch. I drive a regular cab truck so 10'' is the best I can do without giving up any leg room. Thanks for the info Dustin. I think im still going to be buying it because the guy I'm going to get it from is going to sell it to me for only 150$ and its only been used maybe 4 hours tops.
Will it be louder than the D3 with the same amplifier?