From the link above [
https://www.dhtrob.com/overige/tubefriendly_lsp_en.php ] this seems helpful:
"[F] the purposes of determining tube-friendliness, sensitivity is essentially meaningless. Many tube gurus insist that one must use speakers with sensitivity ratings of 90 db or higher. This is simply not true. Many speakers with high sensitivity ratings are poor choices for tube amps, just as many speakers with low sensitivity ratings often work quite well….
Tubes like a resistive load - it’s really that simple….[F]rom a tube’s point of view is as follows: tubes like consistency. They prefer a speaker that doesn’t swing from 8 ohms to 2 ohms and back again….Tubes don’t like to pump out current at the drop of a hat. They prefer a consistent and predictable load.
Naturally it is sometimes difficult for speaker manufacturers to keep all the aforementioned variables in their ideal ranges. If phase angles must be capacitive, impedance should be high. The lower the impedance, the flatter it must be, and the less capacitive the phase angles must be. All three variables (slope of impedance, value of impedance, value of phase angle) together determine how good a match a speaker will be for a tube amp.
For this reason, the argument that only speakers with an impedance of 8 ohms or higher will work on tubes is incorrect. There are many tube friendly speakers whose nominal impedance is 4 ohms. If the speaker’s impedance is relatively flat and consistently hovers around 4 ohms, and if the phase angles are only slightly capacitive, or better yet inductive, there is no reason why a 4 ohm speaker cannot perform well on a tube amplifier. Some of our favorite tube friendly speakers are 4 ohms!"
The article concludes by affirming that both metrics matter AND listening matters:
"You can easily tell if a speaker isn’t performing well on tubes by... listening to it! Certainly we encourage you to gather as much data as possible such that you don’t waste your time auditioning speakers which are clearly not appropriate choices for tubes, but when it comes down to it folks the only way to find the right speaker is to listen to it anyway. A speaker that doesn’t perform well on tubes will typically be characterized by one or more of the following sonic traits when connected to a tube amp:
- Dual-mono imaging, i.e. the speakers do not "disappear"
- Unusually poor imaging or soundstaging
- Lumpy, bloated, or "one note" bass
- Lack of control in the bass - excessive boominess or thumpiness
- A "suckout" in the midrange
- Whenever the bass kicks in, the mids go away
- Whenever the bass kicks in, the highs get harsh and strained
- When you hook the speaker up to a solid state amp, the bass gets tighter, the midrange comes back, and the highs mellow out"
The last bullet point here tells me that the Focals are a bad match BOTH for my solid state AND my tubes -- because the Focal's bass sound mediocre on my tubes, and while the solid state tightens up the bass, the mids get grainy and the highs get harsh.
