Current vs. Voltage

Yes, you always have current and voltage in an electrical signal. In simple terms, we can see this via Ohm's Law:
 
V=IR
 
V is voltage, I is current and R is resistance, here.
 
Every circuit has an impedance (you can think of this as a resistance that varies with frequency). If you look at the equation, you can see that, given a resistance (always present, even in a superconductor), voltage and current must not only be present, they are always related to each other.

It is just a guess, but the problem you may be wrestling with is that real world amplifiers cannot always deliver the needed amount of current at all relevant impedances that speakers have. In the commonly held view, we might want to see something like this:

8 ohm impedance, 300 Watts max power, 6.1 amperes current, 49.2 volts (note: power = IV)

4 ohm impedance, 600 Watts max power, 12.2 amps current, 49.2 volts

2 ohm impedance, 1200 Watts max power, 24.2 amps current, 49.2 volts

The above is hard to do because as impedance drops we have to drive a lot of current through the load. That amount of thermal work requires big heat sinks and large power supplies. The above could be called a voltage source because it can deliver 49.2 volts regardless.

On the input side (above is amplifier output stuff), I believe current mode is used as a way to improve high frequency bandwidth.

I haven't reviewed the NAD M2, but it sounds like it simply has fewer gain stages. There is some thinking that fewer circuit stages (i.e. a simpler circuit) is better.

As with most things in engineering there are in fact tradeoffs, and it is very hard to predict from a circuit feature what will work well.

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In a way, yes, but probably not the way you are thinking of it. The cables aren't the secret, I don't believe. Rather, all the pieces are designed for current mode.
 
Krell amps normally (when used with other non-Krell devices) have high input impedance (50k-100k ohms) as is common with many amplifiers. In that situation, the preamp doesn't have to deliver much current.
 
Krell's claim is that CAST integrates source, preamp and if (at least if they are Krell designed) into a single voltage gain stage. I see that in CAST mode, the input impedance drops from say 100k ohms to 70 ohms (not 70k, but 70). That means the source components have to deliver more current.  For reasons I can't recall, current mode operation like this works better (or can work better) at very high frequencies. The power bandwidth of the Evo 900 is claimed to be about 120Mhz, which is exceptional, so there you have it.
 
Can you hear this? That's another question, though I will say from personal experience that the mbl amps that I use have very high power bandwidth (though far less than what Krell claims) and they sound exceptional. Causation or correlation, I'm not sure.

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