Just so that you might be able to get "un-stoopid", here's a link to the official DBX forum. http://www.dbxpro.com/forum/viewtopic.php?f=61&t=959 Quite a bit of reading, I know, and probably can get a bit confusing too. But slowly and surely wins the race. I can't directly answer your question because I have never operated, or even seen a DRPA+.
Just know it has a compressor and a limiter, so when setting it, just make sure it is the limiter. Then it has "overeasy" which is used mostly for tops for the compression to be less noticeable. However, for the subs you want to none of that. Just plug in your amp at the setting you will have it and then measure the voltage (no speaker). You will need a voltage meter.If you do no have one and just want the amp not to clip, then you can do that instead.