impedance vs sensitivity

I think when you look at the sensitivity of a speaker, the amplifier's watts are important but when you factor in the speakers low impedance, it becomes equally important that the amplifier can deliver adequate current rather than just power. For example, the old Quad ELS 57s only required low power (around 25 - 40 watts) but caused big problems for many amps that couldn't meet their current demands into their very low dipping impedance. I had many amps with the correct power ouput pack it in because they couldn't deliver enough current.

What you are saying initially is pretty common if you think about it... The more drivers, the lower the impedance usually resides.

The BW speakers act closer to being a 4ohm speaker than the 8 ohms they say they are... ex the 802N... Focal speakers too have lows beneath 4 ohms. Tyler units in the past have delved low as well... Even my Silverline Sonata IIIs respond better on some amps 4 ohms taps than their 8's.

Damping rate of the amp can indicate a better fit than mere topology... SS v. Hs. I refer to damping as 'control' of the loudspeakers drivers, preventing them from returning energy to the amp. Generally, lots of watts can overcome it too. Others here can explain this interaction between the amp and speakers far better than myself.

I've noticed this relationship just by how much more low level impact and timberal truth there is, and/or how much more the vol knob gets tweaked upwards to get the same SPL or satisfying sounds. BTW I'm not keen on high levels of Sound pressure, or immense listening levels. So I like an amp which doesn't have to be turned way up to sound good.

No, Low power is not enough for low ohm loads they need more power. Tube amps do not do well with low ohm loads unless designed for it. Your bass will suffer if loudspeakers below 4ohms and run on a low to med power tube amp.

Sensitivity and Efficiency are two specs that try to express the same thing from different points of view. The different points of view are the Voltage Paradigm and the Power Paradigm, which are opposing concepts of how to design and test amplifiers and loudspeakers. For a complete explanation see

http://www.atma-sphere.com/papers/paradigm_paper2.html

Efficiency is 1 watt measured at 1 meter. Sensitivity is 2.83 volts measured at 1 meter. If you work the math, 2.83 volts is 1 watt into 8 ohms. Into 4 ohms it is 2 watts, IOW if you have a speaker that is 92 db 1 watt/1M, it will be 95 db 2.83V/1M, if it is also a 4 ohm device.

The Voltage Paradigm, which mostly describes transistor amplifiers, has no use for the Efficiency measurement, which is a Power Paradigm specification. Amplifier power and impedance are not the same thing; if you have a low impedance speaker that is also high sensitivity, a low power tube amplifier will be able to drive it just fine as long as it is able to get a good match to the speaker.

FWIW these days there is little argument for 4 ohm speakers in high end audio. This because regardless of the amplifier technology (transistor, tube or class D), the amplifier will sound better and perform better on higher impedances. Of course, if you are unconcerned about sound quality, and more interested in sound pressure, than 4 ohm speakers will be more attractive if you also own a solid state amplifier. The other argument against 4 ohms is the speaker cable- they are extremely critical for best results on 4 ohms, while at 16 ohms they are not nearly so. Making a speaker to be higher impedance, all other things being equal, is an easy way to make the speaker appear that it is smoother with greater detail, always a desirable combination.

Atmasphere - I read that speakers, before SS became popular, had much higher impedance (32 Ohm or even higher)?