I'm just curious what kind of circuitry might be in the pen-like
NoiseGuard part of Sennheiser PXC 250. The most obvious, of course, is
an amplifier of the ambient sound that comes from the mics and a
pi-delay unit or an inverter to get antiphase signal.

With the NoiseGuard switched off the headphones sound really much
worse than switched on. So, together with the relative high impedance
of the device (300 Ohm) I can suspect, that there is a kind of
amplifier for the signal from the portable.

Any further ideas?