Getting an amplifier to run "per specification" doesn't make much of a difference. You need the ability to drive a power-hungry speaker and for that, you need current. The Yamaha just can't deliver in that regard.

Let's use a water analogy. Consider a water wheel on a river. The water at the top of the wheel is the "voltage". It has the potential (through gravity) to move the wheel. Let's say the wheel is 20 feet high and let's equate that to 20 volts. A gallon of water at 20 feet has the same potential as each of 20,000 gallons of water waiting to push that wheel around. But one or two gallons just doesn't have the oomph to move the wheel. You need the ability to deliver enough current to make that wheel perform. A weak amplifier is like having ten little old ladies carrying water in buckets from a pond to the top of the wheel. Having a strong amplifier is like having Lake Mead supplying the water. When one gallon goes into the wheel, there's another gallon right behind it. Consequently, the wheel will "perform".

Now, putting a resistor in series with the speaker is like partially blocking the chute leading to the water wheel. The ten little old ladies (the Yamaha) can feel better that the water they dump into the chute doesn't disappear as quickly, but the wheel turns even slower than it did before! The resistor is ROBBING the power that the speaker needs. So you either need to get a smaller wheel (more efficient speakers), or get a more powerful amplifier (give the old ladies a break, willya?)

Quote Originally Posted by F1
by adding the resistor the amplifier is loosing half of the power (= become 100W/c) the volume will only reduce by 3 dB which is easily tackled by volume control.
That's really just saying you'll get those old ladies to work harder by flogging them with a whip! You've got that chute partially blocked, letting less water get to the wheel, but the overworked ladies CAN keep the chute at a higher level than before. BUT...the wheel barely turns. The speakers need CURRENT to move those drivers the way they are supposed to be driven. Give them current! DON'T DEPRIVE THEM!