Skyserve,
I don't know much about the 400cx but I had the 350mcx for a few years then upgraded to 450mcx. I can tell you from experience that the 450 MCX is a much better amp than the 350 MCX. The power is difference is suppose to be only a 100 watts but the 450 sound so much better - deeper bass and fuller mid range. The 350 in comparison sounds lean. The 450 is about 30 pounds heavier and operates much hotter. Based on the heat, I would guess the 450 is biased more into class.
The 450 also works well with Audio Research preamp. I used it with an ARC REF 5SE and it sounded very nice. You get the low end of Krell and the vocal and smoothness of tube as well.
If you find a pair of 450 MCX, I would highly recommend them over the 350.