Wire does not hiss. Tubes do. Switching tubes was a good start, but the preamp must have some other problem.
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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!