Benz SLR Gullwing, not sensitive to loading?

You should definitely hear a difference from 100 ohms to 47k. Sounds like something's off in the loading setup, though I can't guess what that might be. The signal loss from 100 ohms loading, on the 38 ohm SLR or LPS coils, is almost 3 dB (versus 47k) - which alone should be extremely audible, and will surely come with frequency response aberrations on top of that.

Signal Loss at 100 ohms:

20 * log(100 ohms / (100 ohms + 38 ohms)) = -2.8 dB

Signal Loss at 47k:

20 * log(47000 / (47000 + 38)) = -0.007 dB

Personally, I felt my LPS to be lackluster with 10x SUTs loading it around 470. It did much better, and came alive with an active JFET stage loading at 1000 ohms.

When I use my Benz LPS with an Aesthetix Io Eclipse dual power supply phono stage I use 47k. This setting sounds best to me. I arrived to this setting after a long period of experimentation. I appreciate the mathematical formulas to generalize on average settings, but, after being in this hobby for such a long time, I have learned that there are sometimes no absolutes when it comes to actual connectivity of a slew of components and the intended sound outcomes due to a confluence of factors affecting the sound. My tonearm is an SME V with analysis plus silver Apex phono cable.

As mulveling stated.  the difference in output level alone should be clearly audible.  If the output level does not change as you apply a 100Ω load you are not making a connection somewhere.  A simple way to test this is to simply put a 10Ω load (or shorting plug) into the load position and that should give a clear audible effect of the loading.  Once you have confirmed you are making a connection then you can address the sonic changes.