OK, I was told back around 1996 when I bought my first valve amplifier that if one was using eight Ohm or higher loudspeakers then one should be using the four Ohm output tap. I have always done this as good measure without completely understanding why but yes, it makes sense that it would reshape the frequency response. I have quite clearly heard a difference in sound between triode and ultralinear but I do not think this is just down to frequency reshaping. What I hear is not just down to effects in frequencies as if that were the case then the same result could be had by using a graphic equaliser. What I have here is a clear improvement in bass clarity and presence. There is so much more detail in the way the bass guitar is being played. Yes, there are also differences that are clearly frequency based as the top is more rolled off and there is less edge to vocals. I just find all of this extra detail and texture mesmerising.