They are a 4 ohm nominal load. But when they crank out the highs that load increases. They're also more of a capacitant load than dynamic speakers. Contrary to what I used to think, and what many say on many chat sites, the panels thrive with tubes. Peter McAlister explained to me that there are solid technical reasons for that. I've found that the winning combo is tubes on the panels and big solid state on the woofers using digital room correction (I use a DBX Driverack PA).