No sound from left channel on power amp

I've got an Audiolab 8000P power amp in bi-amp mode. Just the other day, I noticed that I couldn't hear the "bass" from my left speaker. I ruled out that it was the speaker cuz I swapped them and same thing happened with the other. I checked the cables and source and ruled out that this problem was caused by any of them. So, this was the power amp I'm using for the low frequencies. I can power it up and one channel is fine. Any DIY suggestions other than bringing it in for repairs?

Thanks