Not neccessarily high current amplifier has low output impedance.
It has
a) high current ratings of the output transistors before clipping.
b) high peak short-term current depending on the design of power supplies.
As to high current preamp I see absolutely no neccessity(or it could just only be a marketing hype) as in reality preamp is supposed to amplify voltage but not power. To decrease an output impedance in preamps there maybe the stage with emitter follower that has very low output impedance, has no voltage gain but has a large current gain. Please note that it's not realy neccessary to use an emitter follower in preamps as it also an additional stage, additional distortions and certainly an additional negative feedback. Not neccessarily it will give you a high current since the output transistors are low-current themselves, but the fact of current amplification in that case is also present. Output devices of preamplifier usually work at low signal regions, have load usually much larger then its natural output impedance. The less sencitive power amplifiers(with low input sencitivity) usually need a higher input voltage or input current to be driven with lower distortions.
The impedance relationships are ideal when source or preamp has near zero output and the following component has input impedance near infinite.
Tube preamps having high output impedance usually have a very high voltage gain that gives them even better ability to controll an amplifier.
