Hey Deadhorsescott, welcome to the depot
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, interesting question. It's not a spot that I'd think to put 12AT7s. They are a lower gain (~70mu IIRC, amplification factor ~60) but high current tube, great for a phase splitter driving a tube power stage (if you are chasing output tube distortion/break up) but in the MT200 you are driving a bunch of high speed FETS, not tubes, the FETS probably need detail/articulation not current. However, you'd need to be sure the MT200 power supply is up for the higher current requirement of 12AT7s. If you want a less sensitive (lower gain) input buffer tube for the MT200 you could try a 12AU7 (amplification factor ~20), again I'd have to question why
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(but they wont stress the power supply). Less gain, less dynamic range, less articulation so unless you are blowing out the MT200 input stage (from whatever you are feeding it with), you'll get the best dynamic range from a 12AX7 (amplification factor 100) which the MT200 is designed for. Personally I'd try some Mullard long plate 12AX7s as they are nice and 3D and should open up the amp a bit (great articulation) while having a really wide dynamic range.Anyway, my 2 cents worth