YEs, high efficiency speakers will go louder with less power, but you also have to figure in the bass extension.
Generally, smaller speakers that go lower with less drop-off and at higher volumes to boot without breakup or distortion require a lot more power, as indicated. That's where smaller, affordable, high efficiency, high power Class D amps come in. All the bang needed for most any speaker in a small package that need not cost a fortune in that the AMP is more efficient (as opposed to the speakers).
Smaller speakers that are highly efficient in general will not extend much to the lowest frequencies, where the most power is needed and efficiency drops accordingly. This is basic physics! For a high efficiency speaker to go low, it has to be larger to be able to pressurize air effectively at low frequencies with less power.
This is basic physics and good guidelines to consider when buying.
For prog rock music, you want speakers that are relatively flat well down in to the 40 hz range or so at a minimum, more if you want to catch all of Rick Wakeman's organ, etc. So larger more efficient speakers and less powerful amp or smaller less efficient speakers and more powerful amp, those are the two ways to go. You need smaller speakers with some low end that can achieve some decent SPLs (can't be TOO small and must be well built with good quality drivers to hold up), so you will need an amp that is up to the task. NAD is not a bad choice but I would be considering a 250 w/ch class D amp like those from Wyred 4 Sound possibly for the very best results with most of the smaller speakers that will likely fit your bill.
Generally, smaller speakers that go lower with less drop-off and at higher volumes to boot without breakup or distortion require a lot more power, as indicated. That's where smaller, affordable, high efficiency, high power Class D amps come in. All the bang needed for most any speaker in a small package that need not cost a fortune in that the AMP is more efficient (as opposed to the speakers).
Smaller speakers that are highly efficient in general will not extend much to the lowest frequencies, where the most power is needed and efficiency drops accordingly. This is basic physics! For a high efficiency speaker to go low, it has to be larger to be able to pressurize air effectively at low frequencies with less power.
This is basic physics and good guidelines to consider when buying.
For prog rock music, you want speakers that are relatively flat well down in to the 40 hz range or so at a minimum, more if you want to catch all of Rick Wakeman's organ, etc. So larger more efficient speakers and less powerful amp or smaller less efficient speakers and more powerful amp, those are the two ways to go. You need smaller speakers with some low end that can achieve some decent SPLs (can't be TOO small and must be well built with good quality drivers to hold up), so you will need an amp that is up to the task. NAD is not a bad choice but I would be considering a 250 w/ch class D amp like those from Wyred 4 Sound possibly for the very best results with most of the smaller speakers that will likely fit your bill.