Reply to KRK RP5 Cuts Out

I have just started using Max MSP and I have been following various tutorials. Once of the tutorials that I followed showed me how to create a sine wave and how to allow my self to change the frequency of that sine wave.

From upon using my new toy, I decided to raise the frequency to 15,000hz and then add 1,000hz on top of that to test how far my hearing could reach. At 18,000hz both of my speakers made a 'pop' and then stopped outputting any audio. I then turned my speakers on/off and played some music which was played back at an extremely low distorted quality.

From upon unplugging all cables and then testing again on some music, the speakers were back to normal.

I repeated this process to check that it was not just a "one-off" and the same happened again.

Are my speakers supposed to do this as the KRK RP5 G2 are meant to have a frequency range up to 20kHz. Do I need to get them checked out?

Any feedback would be greatly appreciated.