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!
cheers!