It didn't hurt my opinion of the Krell when I saw a salesman drop on its corner a 400Xi from belly high onto a solid concrete floor. He looked extremely pained - even somehow cut his hand when it slid out and his boss walked in. He picked it up. You could barely see a mark on the corner where it hit and it literally chipped the concrete. They hooked it up, turned it on, and much to all of our amazement it worked like a champ. You would have had to see it to believe it. The build quality is incredible. Of course that doesn't make the sound.
Now that is not built like a brick but more like built like concrete! I wouldn't do that on purpose though. Last time I heard about an amp that wouldn't break even dropping it three floors up was the Bryston 4B-ST.
I do agree with the posts above in that Krell is an acquired taste. I have owned the Krell KAV-300i for a short period and listened to KAV-400xi. To me, the former has an unrefined and harsh sound but punchy and dynamic. Not a good match with neutral or upfront speakers but great with laidback ones such as Sonus Fabers. The KAV-400xi is a much refined unit and does not sound as raw with better control across the spectrum.
I have also listened to the YBA Integre and concur with MFsoa and Wireless in that it is one sweet and musical amp. Matched with my B&W's, the sound was so sweet and musical that at one point I was considering this integrated although my system was more or less settled. Maybe one day when I decide to set up a 2nd system I'll look into getting the YBA.
Getting back to the topic, not being familiar with your speakers, the Krell and Bryston is worth looking at with the former having the advantage in being more powerful. The Naim is also quite good but would have problems driving speakers with difficult loads. I would also agree with several suggestions on Plinius integrateds. Difficult to describe the sound so it would be best if you could listen to one.