Per the Krell web stie 138v peak to peak.
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It is easy enough to calculate, but why do you want to know? Are you going to try to do some welding with one?
The amp is rated at 400 watts per channel into a 4 ohm load. The power formula is:
P=I^2 x R (where: P=power in watts, I=current in amps, and R=resistance in ohms)
400 = I^2 x 4
(divide both sides by 4)
100 = I^2
(take the square root of both sides)
10 = I
(flip the values across the equal sign)
I = 10 amps
If you are driving 8 ohm speakers, then the current would be half of that or 5 amps.
The 400xi might have a little headroom, but not much.
Unlike its big FPB brothers, the 400xi doesn't like to drive low ohm speakers. A friend of mine hooked one up to a pair of Apogee Stages (3 ohms nominal) and the Krell got hot enough that it melted solder joints on the circuit board.
The computation that Audiopath performed does not yield the PEAK current ability of the amp. So don't be concerned with the 10A and 5A values. Peak power (or voltage or current) all depends upon the time element. The peak value will be much higher as the time to sustain the value decreases. It's a matter of heat dissipation.
How long can the MF equipment you quote produce 50A? They don't say do they?
All things being equal (and, of course, they never are), the net weight is still a pretty good indicator of the capability of an amp.