EFFICIENCY

Efficiency is a relative rating. My speakers are officially rated at 90 dB. However, the manufacturer recommends a minimum of 100 watts, and "200 watts for full musical enjoyment". I've tried many different amps over the years and ended up with 400 WPC amps (at 8 ohms, my speakers are 4 ohms).

So, how can this be? I don't know how the 90 dB rating was identified (I suspect from the driver manufacturer, but who knows?) but they are less efficient than some speakers with lower ratings. It could be that more power is needed because of the impedance curve across the frequency band - they dip below 3 ohms at two points. Or it could be the complexity of the 1st order crossover. A friend who knows electronics looked at the crossover board and jested that all it needed was an input transformer and it could be it's own amplifier.

On paper you should get at least 105 dB from a 100 WPC amp driving a 86 dB speaker. Even allowing for headroom, that would be adequate for most of us in an average room. But all this relates to level before clipping, not "final sound quality". I don't think your question can be answered by anything other than a personal evaluation.

I have driven my Thiel 2.4 (87dB/M) to pretty loud levels with a 130W amp and you have a 190W amp. So there is really no issue with regards to not having enough power for either speaker except for the 2.3 Ohms impedence with the CS6.

Other things being equal--which they usually aren't--I'd certainly go for the more efficient speaker. The more efficient, the less power you need and the better potential to accurately reproduce dynamic contrasts. The less power you need, the less the voice coils will heat and the less the speakers will compress the sound. The less power you need, the more headroom your amp will have and--if you like to play loud music--the less likely you'll be to damage sensitive tweeters. Also, the more efficient your speakers, the freer you are to try a wider range of amplifiers. And so on.

In the interests of full disclosure, 35 years in the audio hobby has shown me that I more often enjoy efficient speakers. The four pair of speakers I have range from 92 to 100 dB.

It's also significant that one manufacturer's 86 dB may be almost the same as another's 90 dB. Most don't cite the parameters of the tests that established the efficiency rating they quote. The most important is whether the rating specified is for an anechoic chamber or for typical in-room conditions (the former is typically 3 dB less than the latter). The next most important is efficiency over what frequency bandwidth: the 86 dB speaker that maintains that efficiency over most of the audible spectrum may, overall, be as or even more efficient than the 90 dB speaker that was measured only at 1 kHz or over a relatively narrow bandwidth (say 200 Hz to 6 kHz).

Another issue is what sort of impedance curve is characteristic of a given speaker. An 86 dB speaker that never dips below, say 6.5 ohms impedance may be far less demanding on your amp than a 90 dB speaker that dips to 3 ohms.

As I said, "other things" are seldom equal, but when they are, the higher the sensitivity ("efficiency" is the term we commonly use, but actually refers to another parameter) the better.

Happy listening!

Joel.