@lukaszwk An impedance drop upto 3.2 ohms is permissible for claiming a nominal impedance of 4ohm on the specification (IEC standard).
If the manufacturer claimed his speaker is 4 ohms and his speaker dropped to 2 ohm or 1 ohm, he is also a liar and a fraud. Return it to him quickly and get your money back. Sue him for damaging your expensive amps and the mental stress he caused. Get him...
@deep_333 a lot of speakers have a dip to around 2ohms, some that you and others probably think are quite sensitive, it's just that most manufacturers don't publish the impedance graphs of those speakers, all your ever see is "nominal impedance 4 ohms". No minimal impedance is mentioned either, unless it is complimentary. Usually it takes outside tests to find what those are.

