Tube output sections have different tubey qualities than tube preamp sections. A lot of the serious all-tube connoisseurs tend to prefer the type of compression and drive you get from a pushed output section, rather than the result of a pushed small signal tube (preamp). Some reasons for these differences: Tube preamp circuits are generally single ended and produce more even order harmonics. In bass amps, tube output sections are generally push pull. The even order harmonics tend to cancel out in the output transformer. These comments should be taken as generalities. Preamp circuits have a relatively low current draw even when they are pushed to the limit. So they don't tax the amps power supply. The output tubes draw high current when they are pushed to the limit, and this introduces sag in the power supply. Sag means the high current pulls the power supply voltage down. This sag will have an impact on how compressed the amp feels and also how it transition from clean, to OD, to full-on distortion. The general assumption is you have a wider window where the transition is taking place, but this is not always the case because global feedback has a significant impact on how the amp transitions. Most instrument amps use a global negative feedback loop between the output transformer and the phase inverter. The phase inverter is the part of the circuit that splits the single ended signal into it's push-pull components. It's a small signal tubes, but it's part of the output section. AFAIK, higher amounts of global feedback extend frequency response and tightens up the feel of the amp, but when the amp transitions into distortion, the feedback loop becomes unstable and can collapse suddenly. This causes the amp to transition quickly from clean to distortion and the sound may be rather nasty. Lower levels of feedback reduce the amount of clean power, but the amp transitions more gracefully from clean to OD, to distortion. So choosing the amount of feedback is a compromise.