Hi Tom,
With regard to Massoth's BEMF PWM frequency. Massoth recommends 16 kHz because of the four options their XLS decoders have, 60 Hz, 250Hz, 2 kHz, or 16 kHz, it is by far the best choice for LGB motors. To quote the Massoth manual "Basically the motor runs quieter and smoother when the control frequency is increased." This does not necessarily mean 16 kHz is the optimum PWM frequency, If the Massoth decoders offered 40 kHz as a PWM frequency my guess is they would recommend it over 16 kHz but perhaps not.
The following is quoted from the ESU V5 decoder manual:
"11.3. Adapting Load Control Frequency
In the LokSound decoders load control works with 40 kHz normally. Sometimes it can be useful to reduce this frequency by half: For motors with little power due to high inductivity. If suppressors (such as capacitors, chokes, etc.) disturb load control but cannot be removed (e.g. some older Gützold® locos). The PWM frequency can be set in CV 9 as a multiple of 1000 Hz. The range of 10 to 50 allows frequencies from 10 kHz to 50 kHz."
By the way, I've looked at the motors of many LGB locos in the course of adding decoders/maintaining my fleet dating from early 1980 Stainz to current MLGB production. I can't recall ever seeing noise suppression caps or chokes on the motors. That's not to say they may not exist within the motor shell itself but that would be unusual. I admit that I've never pulled apart one of the Beuhler motors so don't know. Perhaps others can chime in with their experience.