Maybe I didn't convey it correctly. I'm not saying a TVC adds warmth.
A TVC certainly can add warmth, due to ringing caused by the transformer not being properly loaded. That loading changes with each tap but the designer can't predict the load of the amplifier being used so it will only be exact at one input impedance only.
PVCs have the problem where when you turn the volume down from full up, the bass impact is often reduced. If you listen closely, you'll find that the impact it likely affected across the entire audio spectrum. The reason this happens is because the PVC is a resistance in series with the input and so the source can't control the interconnect cable (reduce its artifact) as well as it would otherwise.
You'll also find that you have to audition the interconnect cables to find one that sounds acceptable. If you've auditioned cables before and heard a difference then you know what I'm talking about.
You can see that in both cases, a buffer is handy to prevent these issues occurring. Its possible to make something too simple and PVCs and TVCs are a good example.