Best you can do with the dual 4 ohm sub is to either run it in stereo off the back channel of the 54a or wire it in series for an 8 ohm load and bridge the back channels of the 54a. Either way you will only be getting 100 watts to it.
If you are going to use that particular sub then I would just run the DPA-250 to the sub with a mixed mono signal being feed to both channels and wire the sub up stereo to the amp and leave the 4-channel to run the mids and highs. You're going to get the same power either way and you are better off keep the bass amp seperate for clarity.
On the other hand, if you want more power then get either the single 4 ohm or dual 2 ohm sub and you can pull 150 watts out of the back half of the 54a for a little more punch in the low end.
