A unity gain power buffer!! Yeah, they could sure work for that!
As for the AB amplifier, remember Vacuum tubes are only N channel equivalents, there is no such thing as a P channel vacuum tubes! It wouldn't be a complimentary symmetrical design but it would be Class AB.
@erik_squires4: Actually, the V-I characteristics define the devices distortion and bias point. Loop feedback may drive distortion down to low levels but the initial distortion is caused by the non linearity in the device itself. All devices are non linear to some degree, some far worse than others.