Into the Linn Klimax 500, for example, which has a low load impedance of 7.7k ohms, the BAT's response at 20Hz will be down 1.8dB compared with the level at 1kHz. That Jonathan still found the combination to sound fine is presumably due to this being outweighed by other, positive aspects of the BAT's sound.
This rolloff at low frequencies can be seen in fig.1, where the lower pair of traces was made into a 600 ohm load, which is typical of some professional amplifiers and is a switchable option on the Jeff Rowland designs. Into a more typical 100k load (top traces), the LF response is flat down to the 10Hz lower limit of the graph. At the other end of the audio band, the output is -0.3dB at 20kHz, with slightly more audio-band rolloff into the low load. Channel separation was better than 80dB below the mid-treble, but with a rise in crosstalk to -67dB (L-R) and -76dB (R-L), due to capacitive coupling between channels. This will be subjectively inconsequential.
Read more at https://www.stereophile.com/content/balanced-audio-technology-vk-50se-line-preamplifier-measurements...