Actusreus wrote: "I would like to hear a speaker designer's point of view regarding this common tendency in speaker design. If designers such as John DeVore and Alan Yun (Silverline Audio) understand this quite well, why don't others?"
Because most audiophiles use solid state amps, most speaker designers don't bother with making their speakers tube-friendly. Which is fine by me, because it leaves a market niche for us little guys to exploit.
Paying a lot of attention to the impedance curve reduces your driver choices, and often makes crossover design more challenging. I could probably get away with about 1/3 to maybe 1/2 fewer components in my crossovers if I didn't mind a roller-coaster impedance curve. Also as noted already, high efficiency + decent bass extension = a big box. With audio in general moving towards small-box speakers for aesthetics, big, tube-friendly speakers are going against the grain.
While impedance dips can reduce the real-world power that a tube amp can deliver, impedance peaks can be bad too because of their effect on the frequency response (assuming the speaker was designed for "voltage paradigm" solid-state amps). Let me explain:
Suppose we have an "8-ohm speaker" with twin impedance peaks in the bass region that rise to 16 ohms, and a 32-ohm peak at the crossover frequency of 2.5 kHz. At a level equal to "1 watt into 8 ohms" a solid-state amp will be delivering 1/2 watt into those bass impedance peaks, and 1/4 watt into that 32-ohm peak at the crossover frequency. If the speaker was voiced for a solid state amp, then what happens on a tube amp is this: You get 3 dB more output into the bass impedance peaks (which can make the bass sound underdamped), and you get about 6 dB more output in the crossover region (which makes the speaker sound forward and fatiguing). Your instinct would be to blame the tube amp and conclude that tube amps are "fat and forward" because the speaker sounded fine on the solid state amp, but the real culprit is the speaker's impedance curve.
The approach I take to the impedance curve is to keep it fairly high and with no significant peaks north of the bass region, so that the spectral balance doesn't change with amplifier type. To deal with the effects of the bass impedance peaks, I tune the box considerably lower than is theoretically "optimum", so that the region boosted by the port is moved down lower in frequency. The net effect is more extended bass than we normally would have gotten, without excess upper-bass thickness. This is as close to a "free lunch" as it gets, but it's only a "free lunch" when contrasted with the same speaker on a solid state amp. (Actually my port lengths are user-adjustable, for compatibility with the various amplifier types).
In my opinion the high efficiency and benign impedance curves that make for a tube-friendly speaker are generally beneficial regardless of amplifier type, so the main drawback is the larger box size imposed by the requirement for higher efficiency.
Of course mine is by no means the only legitimate approach to designing a tube-friendly speaker. The designer can let the impedance curve do whatever and simply voice the speaker for tube amps.
I've done some over-simplifying in this post, but hopefully given enough information to illustrate some of the considerations on the designer's side of the fence.
Duke
dealer/manufacturer
