125W @16ohm, what amp?

The specs simply recommend usage of a (professional) amp capable of outputting 125W into a 16ohm load. That’s a hefty amp, i.e. one that specs 250W or more into the usually quoted 8ohm load.

No worries, many professional amps spec well above that output!

Greg is right on.

Yes, you can use amps rated for LOWER impedance with no worries.  It's just very rare to see a solid state amp rated at 16 Ohms just because 16 ohm speakers are kind of rare.  Tube amps may be different.

An amp that does 125 Watts at 16 Ohms would do about 250 into 8 Ohms.

1. I want it full blast, is a 200w amp fine (2x100w)?

If you want to play loud, you want real power. These are sound reinforcement type loudspeakers, they will play loud, not sure if you will want to hear what comes out.