The ideal situation is to have no limitation in terms of thermal dissipation and current availability. This is the case with the 250 Pro where the output power in dual-mono is then 4 times higher than the output power in stereo (the output power P goes from 250W to 1000W as the output voltage U is multiplied by 2 in dual-mono and P=U2/R). With the 220 Pro, the casing is limiting the thermal dissipation in dual-mono mode and therefore the output power is only multiplied by 2. With the 140 Pro, the class D board is limiting the current availability and therefore the output power is only multiplied by 1.5.