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Ah, I thought the word "hotter" eliminated confusion when talking about bias. Because in the case of power tubes, biasing them hotter really does make them get hotter. But here we have a case where the bias voltage moves in the "hotter" direction, but the tube gets colder. 
I'd like to bring up something that I hope is relevant, and Merlin might have an opinion on. It concerns what happens to your tone when you swap out a 12AX7 for a 12AT7.
The 12AT7 is designed for a higher operating current, so the curved "knee" of its characteristic extends up to higher currents than a 12AX7. But when swapped out it ends up running at a similar current to what the 12AX7 did, maybe a little higher. In other words, it's biased down on the knee, rather than properly in the linear region. So I think that causes more even-order harmonics and that's why many players (myself included) feel that it warms and fattens the tone, at the expense of losing some gain.
I'd like to bring up something that I hope is relevant, and Merlin might have an opinion on. It concerns what happens to your tone when you swap out a 12AX7 for a 12AT7.
The 12AT7 is designed for a higher operating current, so the curved "knee" of its characteristic extends up to higher currents than a 12AX7. But when swapped out it ends up running at a similar current to what the 12AX7 did, maybe a little higher. In other words, it's biased down on the knee, rather than properly in the linear region. So I think that causes more even-order harmonics and that's why many players (myself included) feel that it warms and fattens the tone, at the expense of losing some gain.
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