How to accurately gauge speaker sensitivity to match with tube amp?

The Focal isn't crazy low for tubes but it will give them a hard time. You should use the 4 ohm tap with this speaker.
Generally speaking though if you want to get the best out of a tube amp then the speaker should not go much below 8 ohms especially in the bass region! If the amp has output transformers (and most do) its common for the output transformer to lose bandwidth by as much as an octave on the bottom when using the 4 ohm tap (and that's even when its properly loaded with a 4 ohm load). This can cause the amp to have additional phase shift and that is interpreted by the ear as a loss of impact.


So I see two ways out of this if the 4 ohm taps don't do it for you and you want to keep the amp. Either get a set of ZEROs (www.zeroimpedance.com) and see if they can help you out (by allowing you to use a higher tap on the amplifiers) or get a set of speakers that are an easier load.


Its really not a good idea to make an amp work hard for a living. They make a lot less distortion if they have it easy, and that translates directly to 'sounding more real'.

@erik_squires 

 The more variation, the more the impedance curve takes command of the frequency response….

…but look how broad the under 4 Ohms is. That’s what I’d listen for. For the most neutral presentation from these speakers you are going to want a beefy solid state amp.

Great! Very helpful. FWIW, I only looked at the curve after I knew I was hearing something and wanted to understand why. I had seen such graphed curves before, but now they are connecting with my listening experience; they have meaning, now.

@miller

The only measurement you need is sensitivity greater than 90. Then from there you pay attention to listening impressions. Then when you find people describing a sound that matches what you like look for people who are hearing that sound with tube amps…..But on the other hand, maybe you don't care about such things as musical involvement.

LOL! If I didn't care about musical involvement, I'd be a fool. I hope I've not given off that impression with my question. I'm not a specs fanboy by a longshot, and your suggestion is most appreciated. People's taste (and hearing) can vary a lot, and so sometimes that's an X factor which is hard for me to glimpse in the many posts mentioning speakers in which I might be interested. Another tool (like the sensitivity number) was what I was seeking here, but I'll remain open to the possibility that you're right — that more than just sensitivity measurement is worthless and I should read up on listening impressions almost exclusively. (And, of course, all of this is not replacing listening, but getting whittled down the number of things to which I might go to the trouble of auditioning.)

@yogiboy  Thanks— good suggestion about the taps. They're currently on the 4 ohm taps, though.

@almarg That's right — I'm trying to use specs to help delimit a rather wide range of choice. The power of the amps are similar, but I know their damping factors are very different (20 vs. 100, at least). I will look into what other people's anecdotal reports say, too. I never ruled that out, but the metric of speaker sensitivity has been very helpful in narrowing down my choices and that's why I'm looking into related metrics, now.

The dynamic jumps in symphonic music (I spent a day just listening to symphonies) were not distorting with the tubes.

@atmasphere Thanks for your answer. You're adding more technical information that is pertinent to my narrowing down the candidates -- the comment that "the speaker should not go much below 8 ohms especially in the bass region" is very useful, indeed (at least, to me). 

These speakers are trials. They're going back. No cost to me because I can drop them off. The question is, "what's next." Thanks for your help with that project.

1.Room size?
2.Type of music you listen to?
3.Budget?
4.Floorstanders or Stand mount?

Of course, this presupposes that the speakers you've become interested in have been measured reliably and that an impedance curve has been published.

Fritz's standmounts do very well with low-powered tubes (I heard them with a Modwright SET headphone amp!!) but I'm not aware of published curves for the various models.