Eckmiller passive faders W85 vs MR90

[Fok]

I am new to GroupDIY. I like to know if anyone have experience using vintage Eckmiller faders W85 and MR90. A few questions below

1. what is the input and output impedance of each fader？
2. should I connect these faders before or after a Telefunken/TAB V74 tube summing amp ？
3. what are the sonic difference between W85 vs MR90 faders？

 

[[silent:arts]]

1.
W85 Zin = 200Ω / Zout = 200Ω
MR90 Zin = 600Ω / Zout = 200Ω

2.
a V74 is not a summing amp, it is a line level isolation amp / booster