current booster
Generally a tube has a higher current gain than bipolar transistors, including darlingtons, it's effectively infinity as with a FET. Thus I won't follow your principal characterisation which should be said first, cause you raised the question in a very general way.
Generally tubes have a lower transconductance (the same with small signal FET) than bipolar transistors at a given current level. This actually sets a lower limit to the output impedance achievable with tube amplifiers. Other parameters depend on the used tube type and the circuit. As with bipolar transistors, you can extract behaviour at an intended operation point from characteristic data from the datasheet.