A few years ago all the PL products used a mixture of 12AX7s and 12AU7s. Kevin Deal, PL importer in the US, talked the designer into using all 12AU7s because there were more available and many vintage12AU7s were cheaper than comparable 12AX7s.
Someone at Upscale Audio, Kevin's store, could probably tell you why your 100 has a 12AX7 and which brands you might want to try to change the sound a little, if you want to do that.
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