Пока еще не добрались до внутренностей, но я нашел вот такое дополнение в Интернете:
This amp features something they call Direct Power Amp Control which according to them allows one to overload the power amp at room levels.
The Tube 100 and Tube 50 units also have Direct Power Amp Control.
Оfficial Tube 20 page: (сейчас ее уже нет - прим. NC) - Provides rich, thick power-amp overdrive together with full, fat dynamic tube tone at room volume. The Direct Power Amp Control circuitry makes this possible, and makes the Tube 20 a great club and recording amp.