okay depends on a couple of things here.... 1. Can u charging system handle the power u want to send to it? 2. What is the Rms on the Sub and vc config 3.yes mono is fine for one sub..mono are usually more efficient and cheaper in some cases too. 4. ask other opinnions too cuz im still just gettin the hang of everything....
Get a dual 4 and run it at 2ohms. This leaves you space to add another type r and run it at 1ohm for full power. I personally think 870rms is to much power for a type R. But the new versions might be able to handle the power better compared to the 04 models. So IMO, run it at 2ohms and set the gain right. If you feel that its to much for the sub to handle, turn the gain down a little.