I just finished running scenarios thru TubeCAD, using both the 12AT7 and 12AU7 tubes, and, well... it seems that at 300 volts B+, you need to use such low values of plate load to bring the current up that you lose gain, headroom, and signal symmetry, which granted may not be that much of an issue for a push-pull driver tube - you might be better off with a 12AX7 driver. A negative supply for the common cathode resistor might help, but I admit is a little bit of a weird fix. I'm assuming you're using what TubeCAD and I call a 'differential amp' configuration - two common-cathode gain stages sharing a cathode resistor. With see-saws, cathodynes, split-loads, concertinas, paraphases, long-tails, bootstraps, and other such floating around out there, I get a little confused sometimes. I did notice that all the output peak AC numbers are far above any negative bias values that could possibly be in use, so I don't think that driver-tube headroom will be an issue in this configuration at all.