Me too I'm looking with more and more interest to Karan electronics and especially the Kas400, but it's hard to find it in the used market or for an audition. Last month one item at a very reasonable price in Holland was quickly sold. We must admit that Karan Power amps are not exactly "lightweight" ... going into a range of 65-100 kg each... in my situation of upper 5th floor (and at the moment with a lift out of order) It will be not possibile for me to make a try...
In any case here in Northern Italy a reputated audiophile has superb results driving a pair of MBL 101d with a KAS-600...
Did he post impressions online?
What was the price the S400 sold for?
From everything I read it seems the S600 is the amp that really brings incredible performance and is a big step above the S400.