I always leave them in standby mode (rear power switch on, front power button off), which means the Class D solid state portion is powered on, but the 12AU7 input tube is off. Since tubes warm up very quickly (they have heaters!) they are ready to go very quickly. After 5-10 minutes, I notice no difference. Even after 2 minutes hardly any change. It’s the solid state stuff that can take much longer to warm up, but since it is always on in standby mode, there is no necessary warmup cycle.