If the anode voltage is not greater than 200 volts on the input tubes, a substitute for 12AT7 that I found better sounding than the Tele 801s is the Raytheon CK6414. The tube is incredibly built to a high standard, has a slightly lower gain, and was used in the early tube based computers. They aren't that expensive, either, compared the fashion boutique tubes. They have a larger glass envelope than most 12AT7 types.
Of course, if the anode voltage is much higher than 200 volts, which it usually isn't, then all bets are off.
I don't really think of 12AT7 types as being mellifluous, but the 12BH7 types might be. 12AT7 types are usually chosen for moderate high gain and higher transconductance than 12AX7 types for driving the next stage. Mullard type 12AT7 types might be the most warm sounding.