The frequency response of a U 87 amplifier is tailored for the expected uses of the microphone, and according to Neumann's specifications (in copi0081.pdf from the Neumann "Infopool"), in order to sound right the U 87A amplifier (via its measurement input) should measure about -3 dB at 40 Hz and about -4 dB at 16 kHz, both relative to the gain at 1 kHz.
I'm sorry that I don't have full information handy about the exact way in which this measurement should be carried out, but if I recall correctly, the source impedance of the test signals can be critical, and a substitute for the capsule's capacitance must be in place. I suggest making extra sure that you're measuring the amplifier's response in the correct way before you base any circuit modifications on your results.
--best regards