Depends on whether or not a load is connected to the amp in the first place. Plugging a cord into the preamp out doesn't change the power amp's status at all.
For a solid state amp, not having a load is not an issue. As the load impedance goes up, the load current goes down, and the power produced by the power amp goes down. Nothing connected to the power amp output is essentially an infinite load, so the power out is essentially zero. The amp hums merrily along (well, it doesn't actually hum, but you know what I mean), producing a voltage but putting out no current and therefore no power. Never overheats, never gets tired.