The 400 watts isn't an indication of the aural output of the cab, it's the speaker's maximum thermal disippation. So as Bill says above, once your amp is outputting around 100 watts to the cab, more watts will just generate more heat within the speaker's voice coil, not make it louder.
200 watts into two single 15 cabs is a very capable live rig. I.e., the Orange AD200B into two OBC115 cabs (and it's got nothing to do with valve watts).
I would go for a 500 watt amp if I intended to use one or two 410 cabs at high volume as that would deliver around 60 to 75 watts per driver, which feels about right.
(I wonder if anyone has ever really driven 500 watts continuously into a Barefaced One10?)