EVS's web site describes their attenuators as follows:
"The low impedance version works best with solid state amps and can be used for tube amps as well. It has a 1K input impedance at low volumes and has a somewhat higher input impedance at higher volume settings. The output impedance is always below 1K and usually in the 100 ohm range in normal use."
The impedance specs on the Shure are the same. I should add that I have pro audio gear in my chain and directly before my amp, a PS Audio HCA-2 which has 100K input impedance.
"The low impedance version works best with solid state amps and can be used for tube amps as well. It has a 1K input impedance at low volumes and has a somewhat higher input impedance at higher volume settings. The output impedance is always below 1K and usually in the 100 ohm range in normal use."
The impedance specs on the Shure are the same. I should add that I have pro audio gear in my chain and directly before my amp, a PS Audio HCA-2 which has 100K input impedance.