KT,
Yes, just two channels of that amplifer is not enough for a pair of REV8s. However, once bridged into two channels driving a 4-ohm load it will put out 200 watts per speaker as rated. Pasmag recently reviewed the similar XD500/3. The 2 x 75 watt @ 4-ohm channels actually produced 140 watts per channel into a 2-ohm load. That would be 280 watts in the bridged 4-ohm mode. That will definitely wake up the REV8s and significantly reduce the input gain setting. You could add a monoblock sub amp to your XD400/4 or upgrade to the XD700/5 and accomplish the same in a single chassis amplifier.
When adding power they may need to upgrade the primary supply cable depending on what is there now. Don't overlook that.

David
Earmark Marine