Right. First of all, your amp will kick out 800wrms at 4ohms bridged. Two 2ohm DVC subs are not going to show your amp the correct load. You can run one of those subs fine (provided you set the system up correctly, the sub will probably enjoy the extra power). If you want two subs, you need Dual 4ohm coils.
With respect to gains, GET IT SCOPED. I originally set my system up with a multimeter and when I scoped it about a week later I'd still managed to set it up a little 'hot'. Also, you should scope your head units outputs as not many will run at full tilt without clipping.
Hope this helps.