eleonida, to answer your question; as a rule of thumb efficiency drops with impedance. Your amplifier's power increases because current flow increases as long as your hand can pass the current.
As another rule of thumb, any high end system should be able to hit 100 dB. This assures you can handle all peaks gracefully. Most of us never listen this loud. Power doubles with every 3 dB. So if you have a speaker with 87 dB/1 watt/meter, 90 dB requires 2 watts, 93 dB requires 4 watts, 96 dB 8 watts, 99dB 16 and 102 dB 32 watts. This is wonderful except nobody listens to their speakers at 1 meter. Depending on the room sound pressure levels can be an easy 6 dB down at the listening position. So your 87 dB/1 Watt/meter becomes an 81 dB/watt/meter speaker and with 30 watts you are only going to make it to 95dB. 95 dB is a reasonable loud level that many rockers will listen at and if you do with 30 watts you will be clipping the peaks and the sound will be distorted. So, with your amp you should have speakers with a sensitivity of 93 dB/watt/meter or above and there are many available such as the Klipsch Heritage series speakers. If Your Class A Pass amp is a wonderful sounding amp and will drive a Klipsch loudspeaker to very pleasing levels without clipping. You can't expect rock concert levels out of it but those levels damage hearing. If a system has adequate low end you do not need to play at those levels to get the effect of being at a live concert. Unfortunately, most systems do not but, that is another subject.
