Insight on an unique issue faced with an Accuphase pre-amplifier and power amplifier combo

Hello Experts, 

I am looking for some insight and guidance on an issue that I am facing with an Accuphase pre-amp and power amp combo. Here is my set-up and the issue that I am facing. All units are 100V Japanese models and am using an Acupwr step-down transformer. All units have been connected using the Japanese two pronged plugs to the same transformer.

I have a C-2800 pre-amp, A-48 power amp and P300V power amp. I have connected the C2800 and P300V using XLR connects and runs normally.

When I use XLR cables to connect the C2800 to the A48, I run into a myraid of issues. Every time I switch an input on the C2800, there is a popping sound in the speakers. Additionally, the sound output is 7-9db lower than what it is when connected with RCA cables. If I connect the C2800 to the A48 using RCA cables, none of issues exist and everything runs normally. I am totally confused and have not been able to figure out what the issue might be.

Any ideas/ insights will be appreciated.