Aside from the possibility that the older tubes were substantially worn out, such that the difference in gain was very significant, it is hard to account for why the new tubes have more gain (not necessarily more power), yet sound so much less desirable. Is it possible that the tubes are not the same in design or build? The manufacturer could have changed up something between the two batches of tubes.
In some of the amps I've heard that use the KT150, the sound is a touch too brittle and hard sounding for my taste. Perhaps, as the tubes age a bit, they soften in sound and you might be someone who prefers that softer sound.
In some of the amps I've heard that use the KT150, the sound is a touch too brittle and hard sounding for my taste. Perhaps, as the tubes age a bit, they soften in sound and you might be someone who prefers that softer sound.