The 400 is fast, detailed and powerful. Precise, but a little cold. Taut, controlled bass. It's a Krell, you'd expect that. I've got one. Stereophile did a review several months ago. The only knock on the 400 is that the heat sinks are inadequate. So if you're driving the amp pretty hard for a long period, say at a party, you'll get some distortion. I've never experienced this, but I don't have big parties anymore, either.
I've not heard the 300.
I've not heard the 300.