Thanks Greg, that's cool! 150s look awesome. I've heard nothing but praise for the KT120s soundwise.
I know the kt120 draws higher filament current than 6550, probably kt150 does too. Is that why your classic60 can't take them?
I have a friend's VT100 onlongtermloanidlovetobuyitsomeday but wornout power tubes. SED 6550C is discontinued, so I asked Jim McShane what else for VT100, whether KT120 would work, as it does in Ref110 which also used 6550C. He said no because KT120 is too heavy to mount horizontally in VT100! Pisser! He recommends TungSol 6550, says his AR customers are happy with it and he can match them well. I immediately started thinking about how to support the weight of KT120, but I don't really know if the VT100 can source the filament current anyway.