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!
Wire does not hiss. Tubes do. Switching tubes was a good start, but the preamp must have some other problem.
Here's a good test - get some cheap radio shack speaker wire, and connect two runs from your amp to your speakers (sort of a fake bi-wire). Still have the hiss? There's a problem elsewhere, probably tube related. Hiss goes away? Problem with your Kimbers. But I agree - I cannot think of a reason the cables would ever cause hiss. Good luck!
Hiss or hum? If the latter, the Kimbers are picking up stray fields since they are not shielded. Assuming of course that your bi-wire speakers are properly shielded.

Here's a very simple test without any capital involve. With the amp and preamp "on" but no music flowing, crank up the volume at considerable level. Does the hiss becomes louder? Yes, then with your bare hands hold the speaker wires. Does the hum gets louder? Yes, then see above paragraph. No to both, then it is time to buy new tubes!

Thanks very much for your responses.
Have a great holiday!