From my teardown (
https://germanmasterworks.com/publis...-Analysis.html)
"Then there are two switches on the side board (first photo*): S4, the left, vertical one, selects between linear bass reproduction, with a response flat down to the low teens (switch in the up position), and an audible low frequency cut (down position).
The S3, the horizontal switch next to it, should be set to the right ("Normal).
S3 set to left ("Test") position will make the stock M49V inoperable!
The cardioid-only switch S2, which will improve output and s/n by 4dB, is located on the left side of the TOP circuit board (second photo*).
* photos © Klaus Heyne 2021