I completely disagree. In my experience there has never been a SD cardioid capsule that has a sweeter, more complex high end response than the nickel membrane K54.
it's hard to describe sound by words, maybe we are talking about the same thing: the KM54 (I have several of those) does not have any aggressiveness, soft presence boost and tad recessed in the top octave (called air band by most) with an early roll off. Combined with the characteristics of the tube amplifier this sounds softer than most other SDC microphones to me, e.g the KM84 or Schoeps CCM4.I have to correct my above statement about the filter:
there is something in the schematic that might act like a filter, I'm not sure about the amount: C4+R1 form a frequency dependent negative feedback, the corner frequency is 4 kHz. Interestingly the amount of NFB is advised to be adjusted by selecting the value for R2. I could not find any more information about this process.
Little note: from the schematic you can see that it's important to put the calibration input to GND in the cable connector for the microphone to sound correctly. Otherwise C3 cannot shunt the NFB signal to ground, so NFB would be stronger than intended.