Can I run Teufel speakers on an amplifier with 6 Ohm outputs?

The fact that some amplifier manufacturers make corresponding specifications is due to the VDE standard. In order to meet this standard, the devices must not become hotter than 40° C above room temperature, so that no injuries are possible when touching the amplifier. These 40° C are only reached in the limit range, i.e. at very high volumes over a longer period of time. For normal operation, there is no danger of overloading the amplifier anyway. However, the higher temperature that occurs when connecting 4 Ohm speakers does not harm the device at all. That's why you can connect 4 Ohm speakers to amplifiers with 6 Ohm outputs without hesitation.