I'll chime in with a third recommendation for the Stratos Dual Mono. I'm actually the one who purchased the one recently sold and have had it in my system for a week now. Sorry, but I can't compare it directly with the Krell. It does take an authorative grip on my 89db monitors and is fast and detailed, slightly on the warm side with my tube preamp. I'd say a tremendous bargain at the price, even new. The only Krell's I've heard had a more squeeky clean SS sound with gobs of detail and drive (this from memory of a bi-amped Krell system that left a lasting impression). I think that particular system used Krell amps that are well beyond the price of a Stratos DM or Mono Extreme. Have not heard the 400Xi, and my memories are from long ago, so take from it what you may. Unlike Undertow's experience I would say the Dual Mono runs on the warmish side when serving up the tunes. Not class-A hot, but unmistakably warm. For what it's worth you can find more user feedback on the Dual Mono version here.
Marco
Marco