But a 12AT7 will give you 10 times the current vs 12AX7 and a bit less gain (70ish vs 100 ish, remember the article SC posted about phase spliters, but they were mostly chasing output tube distortion. All the gain (tone wise) happens in the preamps, this is not a spot in the circuit you want more gain, you want headroom and "grunt" which comes from the much higher current of the 12AT7 to drive the output tubes.
My TS100 also uses 12AX7s as both buffer and phase splitters; and the model/version I bought has GT 6L6s in it, nice amp, and it's not to say 12AX7s are terrible for phase spliter, just lower gain higher current tubes like 12AT7s and 12AY7s are a better fit for that job.
Hey glad the MT200 is gong well
hybrid land, tubes warming up some transistors and nice and compact. Also well matched to the ADA preamps