An amplifier that has 400 watts per channel at 8 ohms would be a good start, but it depends on how you are wiring them.
If you are wiring them in parallel, you will have a load of 4 ohms on the amp. The wattage would also compound and you could power them with up to 800 watts or so. If you are just going to run 1 per channel of amplifier, you need only find an amp that will produce roughly 400 watts per channel at 8 ohm's.
My suggestion would be to find an amp that has a built in crossover or some amount of digital processing. You will want to cut the low end content going to these speakers somewhere between 60-100hz in order to prolong their life. If they are truly being used as background music playback at low levels, you can go without a crossover of low cut filter, but be aware that running them at loud levels can result in driver failure.
They are only really 100W speakers at best, the 400W is one of those mysterious "peak" ratings that really doesn't mean anything for a plastic BGM install speaker with a tiny 1" dome tweeter.
I've seen that tweeter blown on the 30W tap of the 70V version without any clipping involved.
I wouldn't waste a 400W@8-ohm amp.
A basic little 150W/ch amp (behringer iNuke1000, QSC GX or Crown XTi) will wring every possible dB out of those things with room to spare -without clipping.
That being said, they are pretty good pro-grade BGM speakers with unique (if not a little questionable) styling and rather high efficiency, so compared to home stereo speakers of roughly the same size these will get louder with fewer "watts" needed.
I've only used the 70V ones so can imagine that direct 8-ohm amplification will only make them sound better.
