Pretty much the same current is required to drive both speakers to the same loudness (speaker Y will be louder by about 1.5dB - not detectable).
4 responses Add your response
Speaker Y is harder to drive from an amplifier perspective.
To start on something like this the first thing to do is convert from sensitivity to efficiency. Sensitivity is 2.83 volts at 1 meter. Efficiency is 1 watt 1 meter; into 8 ohms that is also 2.83 volts.
So we can see that the efficiency of speaker Y is 87.5dB generally speaking. Speaker Z is simply 86dB. But being that it is 8 ohms, it will be demanding less current from the amplifier (Power = Current squared x Resistance; resistance in this case being the speaker impedance). Now it will take a bit more power to drive speaker Z but only 1.5dB more at full power. The only downside is such low efficiencies demand a lot of power and its hard to find power amps that make that kind of power that also sound like music (as opposed to electronics). The rise to 10 ohms is helping out in the efficiency department BTW so the output difference between these two might be more like Kijanki stated, although speaker Y is 'easier' to drive.
One thing not mentioned is phase angles. But quite often lower impedance speakers have harder phase angles associated. Phase angles means there is a bit of inductance that causes the voltage and current peaks to no be in phase. The more they are off from each other, the harder the speaker is to drive.
All amplifiers make more distortion into lower impedances (in many cases interpreted by the ear is brightness and harshness) and this should be considered as well. If you want the amp to sound smoother and more detailed, a higher impedance load (if weirdly and somehow, all other things are equal) will do that for you.
One thing not mentioned is phase angles.
Oh, I did that in a reply to you just recently, in another thread.
Take out the passive cross-over from the equation, and go active. New challenges, sure, but a range of obvious advantages as well.