Power amp > speakers (Impedance matching) ?

[AlainGr]

Some time ago, I bought a Spectral Audio on the used market. The model is a DMA-180, rated at 200w/canal through 8 ohms. At 4 ohms it outputs 400w.

It did not seem to be a problem for me, but recently someone brought the question about impedance matching. His argument is that from the impedance plot of my Tannoy System 15, there could be "hot spots" (the impedance can vary between 8 and 4 ohms depending on the frequencies...).

... Spectral Audio and MIT cables have been working hand in hand for decades now. In the owner's manual, there is a clear indication that the MIT cables are "mandatory" to address the broadband of the output (can go up to 1.8MHZ). There is a warning that the power amp could oscillate if not properly matched with the speakers, they even advise not to leave the power amp ON when not using it.

I have been living for the last year and a half with the Spectral Audio, but that association (between SA and MIT) has always been present in my mind. Now I would like to do something about it. Wouldn't it be about the impedance variations that the System 15 speakers show, I would not mind about this (all looks ok and I never felt there was a problem). But sound wise, I don't really trust my ears for potential variations in SQ.

My question is: does a power amp really change its power ratings when music is playing, because at some frequencies the impedance changes ?

I am actually using a bi-wire set of speakers cables (Canare quad cable), from Blue Jeans Cable.

Thanks.

Alain