Crazy Suggestion since they are hard to find and long out of production but perhaps the 400 watt Melos monoblocks(triode output). They easily put out 400 watts(under rated, especially earlier versions) with a relatively high damping factor too(20 if I recall). The big problem is taking care of them and rare tubes(EL519).
Tubes and less sensitive speakers
I love the sound of tube amps but I use Maggie 3.7's. I have tried an 45 watt tube amp and while I loved the clarity of midrange, I missed the low end slam, even with a sub. Is there a tube amp out there that matches well with Magnepans? I am using the Hegel H360 and a friend recommended Coda high current amps but I still am thinking tubes.