400 Watt-HiFi-Stereo- Power

How many watts do I need HiFi?

If you like loud uncompressed music and your speakers are 90dB efficient, 200 Watts is likely plenty of power for you.
If you only listen to light classical, jazz and don't expect them to rock the house, 50 Watts is adequate.
Higher efficiency speakers, like horns, take very few Watts.

How much power do I need from a stereo amplifier?

If you are playing light dance music, the amplifier's 4-ohm power should be 1.6 x 100 W or 160 W continuous per channel.
To handle heavy metal/grunge, the amplifier's 4-ohm power should be 2.5 x 100 W or 250 W continuous per channel.

What size amp do I need for 400 watt speakers?

Considering wattage of each speaker is 400 Watt, you need a stereo amplifier with a rating of 300 + 300 Watt (minimum) that means 300 W of each channel (left and right).
So. one stereo amplifier having output of 300 Watt (minimum) is sufficient to drive both speakers.

You'd be surprised by how little power most speakers actually need to sound decently loud.
Just 10W to 20W is more than enough for typical loudspeakers in modest listening spaces.
Generally speaking, a 50W amplifier is more than enough for home listening with plenty of headroom.