Smoke on my power amp

I was listening the music with the right speaker wire detached to listen to the left speaker alone. The speaker wire was still attached at the amp side. My preamp does not have the balance knob.
After 20 secs or so, there was a smoke on my power amp.
I quickly turn off the amp.
Once smoke comes, would it be safe to turn it on again? I wonder whether it is permanently damaged.
Is it not safe to listen to the music with the speaker wire detached from one speaker but still attached on the amp side?
My amp is Plinius SA 102.
Any comment?
thx