Hi Dan...
There's one thing for sure... you'll have an awesome sub when you are done...
Have you thought about a BFD? If you plan on adding one then you will want to plan cables according.
I suggest using an RCA to XLR to run from your sub pre-out to Input 1 on the BFD. Then from the BFD Output 1 to Input 1 on the EP2500 use an XLR to XLR cable.
When setting the DIP switches... DO NOT pay attention to the labeling above the DIP switches themselves...
...they are not labeled correctly.
Instead look at the manual or the label on the right side of the back of the amp away from the actual DIP switches and set them as follows...
You will actually only use one channel of the amp. Wire your sub in series for a 4 ohm load to the amp, which will give you 650 watts for the sub. If you ever decide to add another sub... you'll have another 650 watts sitting there for use... just connect a speaker cable to it, nothing else will have to be done.
Keep us posted on the project!
I did change your thread title to "Sonotube RL-p15 Project with questions..." and moved it to the SoundSplinter forum as well.