Bi-amping????

Passive biamping with a 100x2 amp is not like sending 200 watts to your speaker - it's merely a very common misconception.

The usable power going to your speakers is really still only 100wpc since that is the maximum power dictated by the voltage swing available from the amplifier.

Now, OTOH, if these amps can be bridged you would have a meaningful increase in output voltage swing as bridging operates the channels inverted from each other. When the channels operate inverted you get up to twice the voltage swing and twice the voltage swing means 4x power - provided the amp's power supply can keep up, that is.

The reason I question the power to the speaker is because Reproducing musical signals, the NAD 2200 routinely delivers over 500 watts per channel into typical
loudspeaker impedances the 2200 is a very conservatively rated 100 watts/channel power
amplifier that has an extraordinary +6 dB of dynamic headroom, meaning that it can produce more than
four times its rated power during musical transients.the unique PowerTracker control circuit
(patent pending), which automatically adjusts the amplifier’s maximum power output according to the
dynamic character of the signal that is being amplified. As befits a product designed for the
reproduction of music, the NAD 2200 achieves its maximum power output of 400 to 800 watts per
channel when amplifying wide-range musical signals that contain peaks IO to 20 dB above the average
level. But when the amplifier is fed a high and constant signal (i.e. a sine-wave test tone instead of a
musical waveform), its maximum output automatically declines to avoid overheating, and eventually
levels out between 100 and 200 watts per channel.

As far as the power it has two power supplies, switching to the
high-voltage supply when maximum power is needed, and switching to the lower-voltage supply for
cooler operation at average power levels. (The switch is called a commutator; hence the name for this
type of amplifier.)

What is the difference between passive biamping and vertical?

Passive biamping means that you still use the speaker's "passive" crossover. The amps receive the full-range of the signal from the preamp and feed this to either the mid-high or the woofer section of the speaker. The spkrs' internal crossover then attenuate part of the frequencies as required.

"Active" or "line-level passive" crossing is when you split the signal BEFORE the amplification channels. Then each amp channel amplifies only the frequencies required for the drive units it's connected to -- i.e. low frequencies go to the woofs, etc. This is much better as the amp is directly connected to the drivers.

"Vertical" is just a way of connecting the amps to the speakers -- 'vertical" being when you use one channel for bass and the other for the mid-high section of spkr (i.e. it's vertical).

I take it that active biamping is better because it is less work for the amp because is not having to reproduce full range?

Would you need a certain Pre-amp with that capability?

In most general discussions, passive biamping refers to using the speakers internal crossover vs an external crossover - although there are external passive crossovers too.

Vertical biamping refers to using the left and right outputs from one amp driving their own section of one speaker.

Horizontal biamping refers to using the channels of one amp for the lows of the left and right speaker, and then another amp to drive the mid/highs of the left and right speakers.

Don't worry about the power, you are making much ado about nothing. The cardinal rules are turn it down if the speakers start making strange noises, or if the sound takes on a harsh and or flat, non-dynamic sound.