Depends on the efficency of the driver/horns and the SPL level you desire. I am running 5 watt battery P/S gainclones on my horns using the Fostex 208e sigma and a ribbon tweet. Use to be a 206e and i was running 24 watt SMPS P/S gainclones and found the volume position to be a bit touchy. The SPL would have a tendancy to jump up or down according to the recording. I have a far greater control and a much more tube like sound at the lower voltage P/S. With the efficency of the horns i really dont believe more than 10 watts is necessary, but thats MHO. ron