100 W Tube Amplifier Malfunctioned

I will not name the brand of the malfunctioned amp in question, except to say that it is a US brand and it is not a boutique shop. I bought direct from factory as there were no dealerships in my region of the world, Taiwan. As I traded in my old amps, I was given a discount and paid $5,000 for the pair. 

My system consists of a Verdier Turntable, A MFA Luminescence preamp, a Wadia 16 CDP and Eminent Technology LFT 8b speakers. The amps were brand new and worked well for 6 months. Then one of the amps malfunctioned. I sent it to a local technician, and determined that the transformer was shot. 

The malfunctioned amp was sent back  at my expense. After inspection,  I was told that the amp failed because it was operating over voltage. Our power grid delivers 110 v electricity.  And in order to be certain, the other amp which was perfectly fine was requested to be sent back for inspection. 

The solution for repair was to increase the VAC of the amp to 125 V, obviously for both amps, and the tubes will be inspected and biased accordingly. The cost for repair will be $ 850 for each amp. My questions is: Should I pay for the repairs? I was told the reason for the failure was due to the fluctuation of our local power grid. Therefore, the user is at fault. However, if it is  power grid is the problem, then why did only one amp fail? Would it not be a reasonable assumption that there are quality control issues at fault? 

Given that there are brand new amps ( six months old ), isn't paying $ 1650 for repairs unreasonable? Any insight would be appreciated.