There are two main groups of power amplifiers in an NVA system:
Stereo models are the most common and integrate power supply and two channels of amplification in a single unit. This setup also forms the entry point into an NVA pre/power system with the P50 preamp and A60 power amp.
Starting with say a P50 preamp we have three sets of outputs per channel, so we can not only support bi-amping but also tri-amping for speakers with this capability. Lets suppose we start with a P50 and an A60 amp and a pair of speakers that support bi-amping (most modern speakers).
We can upgrade our system at a later date; for example we add can add the A70 monoblock power amplifiers:
Here we have the A60 driving just the tweeters, relieving it of the low frequency current needed to drive bass units. The result is lower distortion, more detail and sweetness. Each A70 drives only a single bass unit and much less crossover. This results in much less intermodulation giving better bass and midrange.
The A70 and A80 power amplifiers are actually two boxes, and come in matched left/right pairs. This has several advantages each channel is completely independent of the other, and results in less interference between left and right channels, giving better stereo imaging. But the greatest advantage is that each channel has its own power supply, or dual power supplies in the case of the A80. This results in greater dynamics and incite into the music.
Note: not all speakers can be multi-amped, some have only a single pair of binding posts on the back. So in this case we can not obtain better sound with multi-amping. Instead we must simply use better amplifiers, for example a pair of A80 monoblocks:
You may be asking yourself this question, "What's better A60+A70s or a single pair of A80s?". Well unfortunately there is no easy answer as it depends on so many variables. But I can give you some guidelines. If your speaker has a complex or high-order crossover, or has a uneven impedance curve (especially around the crossover point) then multi-amping is probably the better route to take. In fact I would recommend the multi-amp route for speakers with 3rd or 4th order crossovers, as each amplifier then `sees' less crossover. If your speaker has a simple (low-order) crossover networks, and even impedance curves then I recommend either solution.
Of course there is a limit to multi-amping: