400w rms would be a cvr.
They are dual voice coil subs, 2 or 4 ohm.
The 4 ohm version seems to be a lot more common, but w/o knowing for sure it's hard to recommend an amp.
D2's would actually be preferable IMO b/c you'd have a 1.3 ohm wiring option. You'd want an amp 1000+ at 1 ohm.
The D4's would get you 2.7 ohms so you'd want an amp that makes 100+ watts rms at 2 ohms -- which tend to be more expensive than a 1 ohm amp.
You'd definitely want a mono -- especially since it is now known you 400w subs instead of the 150w Comps.
For one - a lot of them are class D which makes them more efficient than the typical 2 channel amp (class a/b).
And two - working with these unusual impedances (due mainly to 3 subs vs 2 or 4) is a lot easier to deal with using a mono.