The DC-coupled cathode follower stage (V1b-V2a) has a unique overdrive quality described in many books and sites. The squishing of half the waveform is perceived as warm or rich, and this stage produces lots of this tonal quality as you drive it hard. A hard-driven V2a produces lots of harmonics for the tone stack to control, and it drives the V2b driver stage into distortion as well. Once V2a and V2b are distorting, it's likely that V3, the power tube, is also driven to distortion. So, getting overdrive distortion from a stock Uni drives all three stages (V2a, V2b, and V3) to distortion at around the same input level or volume setting.