Good question. A general comment. The 12AT7 family has approximately 3.5 times the transconductance of a 12AX7 family. Subbing a 12AX7 for a 12AT7 will mean only 1/3-1/4 the plate current, so idle plate voltage will be much higher than with a 12AT7. Two questions emerge.
1) In an amplifier, is there enough signal voltage swing capability to drive the output tube to full power before maximum plate voltage swing reaches B+, and "flat tops".
2) As the plate current drops closer to zero, the curves become more nonlinear. More distortion.
So no matter what the circuit, conditions won't be optimal.
Cheers.