They are very neutral and musical. I use a pair off ECC803 with Primaluna prologue Two and they make a huge improvement to the supplied 12AX7 valves (Chinese, I think).
Now the sound has a much more refinement and musical character. They are better extended at highs, with much more information. The original valves are a bit congested in the mid to the highs, what make it a little aggressive in some records. Now, the mids are clear and smooth.
Bass is different too. I would say it has a better tonal balance.
They are expensive, but once you customize to the better sound, you don't want go back.
