You don't say if you are using kt120 or kt150 tubes. Are you using matched pairs of tubes? I use matched quads that are hand matched after they are burned in a bit. This gives you much closer matched tubes. This I the same thing Audio Research does to match their tubes.
ARC Reference 150 Tube Biasing Issue
Installed new tubes for my ARC Ref 150, and the bias voltages look pretty weird. When I set the bias for 65 mv on the power tube, the slave tube shows voltages in the NEGATIVE 65 mv range. Two of the tube pairs do that. The other two pairs show voltage around NEGATIVE 90 mv on the slave tubes. Any one have any ideas what might be causing this?