Can someone explain hi-current preamp?

"High current" is another way of saying low output impedance. Even with a relatively low 10K impedance input of the power amp, a 1 volt preamp output signal would require a current of only 0.0001 amp.

Low output impedance makes the preamp-to-poweramp wiring less sensitive to noise pickup. Solid state preamps have output impedance in the range of 10 to 50 ohms, whereas tube preamps typically run about 600 ohms (although some are lower).

The take-away point is that with a solid state preamp there is much less reason to invest in very expensive interconnects.

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.

Marketing or ignorance. Take your pick-they seem to run hand in hand.
Curt