attached is what jay is saying, but using pictures.
green line is my best guess at what you have now. a small 8" with 100 watts.
red line is the dvc 15" in 3 cubic feet, sealed, 500 watts.
white line is the dvc 15" in 6 cubic feet, ported, tuned to 20hz 500 watts.
edit: the light blue line at the top is a very rough approximation of the f20 output with 500 watts. you can see why it was suggested earlier.
all with a 2nd order high pass filter at 20hz (your plate amp probably has something like that built in).
depending on how large the room is, you will get some boost below about 25-30hz or so.
so the small sealed 15" with 500 watts is 'more gooder', but if you can figure out a way to make a larger ported enclosure blend into your room, it is 'more more gooder' and if you can handle the shear size of the f20 horn sub, it is 'more more more gooder'. these are the tradeoffs.