It is hard to give a minimum figure because of the factors mentioned in my first post. As Nick points out the impedance can go quite low, look for an amp that increases its power at low impedances. The M3 is rated at 275 watts at 8 ohms but on test it produced over 800 at 2 ohms with one channel driven. The TriVista, Kw500 and Kw550 all have even more power. These all have nuvistors [very long lived tubes from the military] in the front end. The 308 basic amp has gotten good reviews , it is similar in design without the tubes.
