@viethluu @jjss49 is correct, it appears that the KEF loudspeakers are not a good load for this particular tube amp. If you look at the speaker you see that has a dual woofer array- so its nominally a 4 ohm load in the bass.
For this reason you should be using the 4 ohm taps IMO/IME. But its a simple fact that speakers designed this way are not intended for tube amps, which don't do well on 4 ohm loads. This is because the output transformer is less efficient on 4 ohm loads and not only will run hotter (absorbing amplifier power) but it may well lose as much as an octave of bandwidth on the bottom end!
If you really want to hear what tubes are about (and to also get lower distortion out of your Krells, which won't hurt) you would need to have a speaker that is at least 8 ohms in the bass to make a more valid comparison.
Put another way I suspect that because of the load of your speakers you are for more likely to get good results from solid state than tubes, but even solid state suffers because while it can drive the load easily enough, to do so it makes more distortion, which obscures detail (including soundstage information).
For this reason you should be using the 4 ohm taps IMO/IME. But its a simple fact that speakers designed this way are not intended for tube amps, which don't do well on 4 ohm loads. This is because the output transformer is less efficient on 4 ohm loads and not only will run hotter (absorbing amplifier power) but it may well lose as much as an octave of bandwidth on the bottom end!
If you really want to hear what tubes are about (and to also get lower distortion out of your Krells, which won't hurt) you would need to have a speaker that is at least 8 ohms in the bass to make a more valid comparison.
Put another way I suspect that because of the load of your speakers you are for more likely to get good results from solid state than tubes, but even solid state suffers because while it can drive the load easily enough, to do so it makes more distortion, which obscures detail (including soundstage information).