In the new 2020 version of the DISTORTION UNIT (left) the input impedance of the balanced line is 10k (instead of 50k) and the unbalanced line input impedance is 100k (instead of 50k).

The input stage tube is a 12AX7/ECC83, it can however be replaced with the 12AY7/6072 for a more transparent tone (lower distortion at low signals) but once the 12AY7 is driven hard the distortion builds up quicker and is harsher than the distortion/coloration of the 12AX7. Most people prefer using the 12AX7.

This unit has no UL (Ultra-Linear) mode. The power supply is inside the unit.

This DU version has a transformer that saturates at high BIAS settings and generates its own coloration/distortion in the low frequencies below 60Hz and especially around 30-40Hz.

Therefore, for a cleaner sound in these very low frequencies, the BIAS control must be set around 3.5 in TRI-L mode and 6 in TRI-H mode.

By keeping the BIAS control low, the signal threshold where grid current distortion kicks in will be increased.
In TRI-L mode, however, unlike the older versions of the DU, the grid current distortion always appears first even if the BIAS control is set at zero.

In the latest versions, a low-pass filter has been added in the output to smoothly roll off the high frequencies in pentode mode, because this mode boosts the treble frequencies disproportionally.

This particular output transformer generates a lot of coloration in the low frequencies and the treble lift in pentode mode is part of it.