One possibility. If these new speakers are 2-3db higher in sensitivity you may be playing them louder than the Kefs not realizing you need to lower the volume. Louder than normal levels can make mids and vocals a bit sharp or harsh.
Happened to me once. Upgraded speakers. Was testing them out on familiar music, but not remembering that I should have reduced the volume about 35% due to 3db increase in sensitivity. Once I realized this and lowered the volume I was very pleased with the sound which had sounded occasionally bright or sharp when I had the volume too high.