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

I'm in the process of matching speakers to my amplifier and need a bit of advice. Most recently, I'm trying Focal 936 towers with my Quicksilver Mono 60w amp. They were sounding pretty decent until I experimented by hooking up my old Adcom 535L amp. All of a sudden, there was a giant jump in control, tautness in the bass, quickness in transients. The QS stuff was doing quite decently, but the Adcom really snapped these towers to attention. The mids and high ends, not to mention the soundstage, were worse with the Adcom — no question. But there was quite a difference with the other qualities just mentioned.

My question becomes one of sensitivity. The Focals self-rated as 92 db. Stereophile rated them as 89.5db. I realize that these are average measurements and a much bigger picture is told by the impedance graph (and other factors).

As I continue to search for the right match of speaker (I have a couple contenders), I'm sure one piece of advice is to look for speakers with higher sensitivity averages. But what else should I look for to help make a guesstimate about whether the amp will drive the speakers with the kind of control they are capable of? [Specs for this amp are here: ]

I realize I need to hear speakers, in my house, with my gear, etc. to get a sense of them. I’m working in exactly this way. Your advice can help me eliminate candidate speakers that would pose similar challenges to my amp as these Focals have.

And I just bought the amp, so I don't want to change it.

Thank you for any thoughts. 

P.S. Anyone who has has had great success with this amp or similar, please shout it out.

Adcom amps typically have a very high damping factor.  They are not necessarily the "sweetest" or "smoothest" on the upper-mid and top-end range, but they definitely drive hard and make for some impressive bass control.  A high damping factor will provide very good control of the bass drivers.  That is the very taught bass experience that you described.  I clicked on the link you provided for your amp and read through the specs.  It indicates that your amp has a damping factor of 20.  If the printed specs are accurate that is a very low damping factor and is likely the reason your speakers sounded as you experienced.  

I don't mean to burst your bubble on that amp, but that is what I see in their specs and that is what I understand (and have experienced many times) with a low damping factor.  If you really wish to keep that amp then a different set of speakers is likely in order.  Some of the options and choices mentioned above certainly make good sense.

I wish you good luck in your quest.  Be safe and be healthy.
@mammothguy No bubble burst; I knew their on-paper damping factor but reports about their abilities were pretty positive. And they do a very good job, but I’m totally experimenting with speaker choices here and it cost me nothing to learn something. But I think that damping factor and these speakers provides something of a data point going forward. Appreciate your thoughts and please take care.
Again, David, keep in mind that the QS’s damping factor is higher than it is for many and I’m pretty certain most high quality tube amps. Assuming the spec is accurate, for the 4 ohm tap a damping factor of 20 means an output impedance of 4/20 = 0.2 ohms, not too far from solid state territory.

Although many audiophiles believe differently, Ralph ( @atmasphere ) and some other technically knowledgeable members (e.g. @kijanki) have explained in past threads that no speaker in existence requires a damping factor greater than the mid to high double digits, and perhaps even lower. Perceived differences in bass response and control between damping factors of say 100 and 2000 are due to other differences in the designs of the amplifiers, IMO and theirs.

Best regards,
-- Al

" A high damping factor will provide very good control of the bass drivers."


For a quick summation of my thoughts, skip to the last two paragraphs. Apologies for getting fairly nerdy in between here and there.

In practice, any series resistance in between the amplifier and the woofer’s voice coil effectively ADDS TO the amplifier’s output impedance, and correspondingly reduces the damping factor.

Let’s run some numbers. Supposes our speaker has a nice 2.5 kHz second-order crossover, which calls for a 1 mH inductor in series with the woofer. Power handling requirements are easily met by an 18 gauge air-core inductor, which can handle 300 watts before saturation. The series resistance of this inductor is .51 ohms.

And let’s suppose we have an uber-amplifier with a damping factor of one zillion. Or one zillion zillion. Or one zillion to the zillionth power. It won’t matter.

After the signal passes through that inductor, our uber-amplfier’s amplifier’s effective damping factor is now about 17. And this is assuming only the one series inductor, and ignoring any other wiring.

So in most cases it really doesn’t matter how high the amplifier’s damping factor is. The series resistance in the crossover (and/or speaker wires) dominates.

Okay, but what about this "very good control of the bass drivers" that we’re apparently missing out on?

Well, turns out that it’s not nearly as dramatic as the wording implies. It all shows up as a change to the electrical damping of the woofer’s motor - the electrical system Q, or Qes.

Assuming a typical high-quality 8-ohm woofer in the example above, the series inductor effectively raises the woofer’s electrical Q by about 7%. So if the woofer’s electrical Q was .28, the series inductor effectively raises it to about .30. This could EASILY be an improvement!  We'll get more bass with a higher Qes, but the designer should take it into account by sizing and tuning the box based on our modified Qes of .30. And if he hasn’t, this difference can still be largely compensated for with a few handfuls of stuffing material.

I think amplifier marketing departments may have oversold the benefits of having a high damping factor.

Or to put it another way, in my opinion, super-high damping factors are, in most cases, of academic interest only. I certainly would not trade off anything that really matters in order to get a high damping factor.