Most amplifiers drive 4-ohm minima though low-impedance loads trigger limiting

Evidence

Power amplifiers should be constant-voltage sources—the output voltage should be independent of the load impedance. There are limits, though, relating mainly to the current available from the power supply if the load impedance is especially low. Occasionally, an unfortunate interaction between amplifier and loudspeaker provokes current or voltage limiting or the activation of a protection circuit, all causing momentary audible effects during crescendos. Most power amplifiers, including many receivers (check the specification for minimum load impedance) can drive a minimum impedance of 4 ohms comfortably, and some can drive 2-ohm minima. Unfortunately, it is not always specified. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1144