According to the serial number, the preamps were from the same batch. However, each had its 'features'. The first one, with the TREBLE knob, boosted high frequencies, while the BASS knob... (drumroll) also boosted high frequencies. The second one seemed to work fine, sounded quite nice, but emitted a constant low-frequency noise. Not 50-60-100 Hz, as it sometimes happens, but just noise at low frequencies. Returned both, thanks to Thomann for their promptness.