The speaker max power handling is basically determined by how much juice the drivers can physically take. One reason for this rating is to warn you in case you decide to, for example, fill an auditorium-size room with very high SPL levels which requires a lot of power. The power level required to reproduce what you want may be in excess of what the speaker can handle. It's your room and the sound levels you want to produce therein that determines how much power you'll need.
For most home listening room environments, about a tenth of a typical SS amp's power can produce more than enough sound levels for most types of listening. Rarely will you need to crank up the volume to the point of 100% of amplifier potential. Even though your speakers may be rated for 200W maximum, a 400W amp can be attenuated down to a fraction of its rating, so it will not harm your speakers. The only time this could be a problem is if the volume knob is unknowingly pegged full when you start a CD. This type of damage is, I think, very rare.
As far as your speakers being 6 ohm and not 8, this can be a little misleading. It's just an "average" impedance. The actual impedance will most likely vary at different frequencies. Better for an amp to see a flat impedance at 6 ohms instead of one of "8 ohms" that fluctuates all over the place between 1 and 8 ohms. Personally, I would not consider it a factor that an amp has a greater power rating than the speakers can handle - I'd never use it.
