Amp problem

A few months ago I wanted to see how balanced cables would sound going from my pre to power amp.

So I got some XLR cables, plugged in, every thing was good, though not really a big change. 

I then received some RCA interconnects that I wanted to try out.

After connecting them I now get a pop from one speaker when I power up the power amp, the volume from that one speaker is a fraction of the other speaker and there is a hum coming from it. The only remedy I have found is to engage the phase inverse on the pre. The hum is greatly reduced, and both speakers play at equal volume. Also if I engage the mute button on the preamp, I do not get a pop in the speaker when I power on the power amp.

Plugging the XLR cables back in returns everything to normal.

After testing multiple sources and swapping wires around, it seems like the problem is with the preamp. If I swap the output wires on the rca outputs from the left to the right channel on the pre, the problem moves from the left speaker to the right speaker.

So my question is, short of taking it to a tech. Is there something I can check in the preamp that I can possibly fix myself.

No I am not a technician, no I do not want to get electrocuted.

Thanks for any insight you can provide. I enjoy the forum.

Rob

System 

Thorens TD 145

K+K phono pre

VTL TL-5.5 (preamp)

Threshold T-200 (power amp)

Proac response 1