Bed QC of Optoma Nuforce MCA-20 power amp

Hi guys, I come from Hong Kong. I just want let you to know, I had very bad online shopping experience through Amazon US.
Nov 2017 I through online shopping by Amazon US bought Optoma Nuforce MCA-20 8 ch power amp. And recevied before Xmas. I have not open it, because i wait new Marantz pre amp 7704. Until 5 Jan 2018, my new Marantz pre amp arrived, I connect with Nuforce MCA-20 power amp to test. I hear 1 ch (Sub ch) sound is too low and different. I disconnect all ch and reconnect all ch again. Sub ch was not work and no sound. I through email to contact Optoma, but they have no reply, but I think my unit must under warranty. I can afford this lost, (total lost) But I want you to know, Optoma products QC and customer service was very bad......very bad...............Very bad So all US customer please becareful