From biwire to single run: Now I hear a loud hiss.

If you guys and gals could help with this I would certainly appreciate it because it's driving me nuts.
My system has always had the speakers wired in a biwire configuration until recently.
After selling my biwire cable I went, for the first time, to a single run config using my old trusty Kimber 4TC that has been sitting in storage for years.
Jumpers are the Kimber 4TC cable.
All connections are bare wire.
As mentioned above, I've been hearing a loud hiss when I power up ever since making this change.
I checked and rechecked everything-cables aren't contacting other cables and power cords, connections are right etc.
Could it be the cables?
Could it be my tubed preamp?
I've already switched tubes and the hiss is still there.
Please advise!
Thanks!