I suspect the problem is a distortion generated by tubes. When we tested tubes, we used to thumb them to check for ringing, a common failing of tubes. But that was in the late 50s when I was a grad student.
I'd use a good solid state preamp or processor, unless you need the warmth distortion tubes can provide. According to the KEF manual, the 107 KUBE is integral to the design of the 107 and required for accurate sound. It provides active equalization for the crossover transitions that a passive R/C network would provide at the cost of lower efficiency. You can bypass the HF and LF tweaking controls, as I do with my 107/2.
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I'd use a good solid state preamp or processor, unless you need the warmth distortion tubes can provide. According to the KEF manual, the 107 KUBE is integral to the design of the 107 and required for accurate sound. It provides active equalization for the crossover transitions that a passive R/C network would provide at the cost of lower efficiency. You can bypass the HF and LF tweaking controls, as I do with my 107/2.
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