One reason SETs suffer their compromised renown is people's insistence to mate them to speakers they shouldn't. There's no blame in that. The parameters that make a speaker copasetic or not for such amplifiers unfortunately aren't positively ascertained just because it'll play loud enough. Loudness doesn't equate to proper drive. If the drive is compromised, you'll get underdamped bass, unpredictable and excessive bloom, frequency domain nonlinearities and a lack of articulation and definition.

What type of specs compromise drive for such amplifiers? Wild impedance swings and large phase angles. Unfortunately, impedance curves and phase plots tend not to get published by the manufacturers. Loudspeaker impedance hides behind nominal, which is to say an averaged value. A speaker that spikes between 1.4 and 50 ohms might find itself called 8 ohms just as a speaker that never drops below 6 nor exceeds 9. Panel speakers in general and many popular dynamic speakers are often designed for the power-is-cheap amplifiers with their massively paralleled high feedback architectures.

One design element of single-ended amps tends to be the avoidance of global feedback. Another is high output impedance. Add those two factors and an amp's ability to deal with "all over the place" speakers becomes severely limited. It may well play loud enough without clipping. That's not the arbiter of good sound, however. The amp's highish output impedance interacts with the speaker's frequency-dependent impedance fluctuations. Now the speaker drives the amp rather than the other way 'round. Remember, amplifier behavior is supposed to be load invariant. Someone's gotta be boss. That's the amplifier's job. Unfortunately, put together a SET
and the wrong kind of speaker and the boss just relinquished control. And out of control is what such a combo will sound like - if you know what it should sound like in the first place and recognize what's wrong with the picture rather than believe it is as it ought to be.

Of course, many SETs are not designed properly to begin with. A giveaway is their THD distortion spectrum which is always tied to amplitude and frequency. Distortion will increase as you're nearing the amplifier's max output power. The question is, by how much. If you play highly dynamic material, instrumental timbres could change rather drastically during peaks. Since it's unlikely that you'll have access to THD plots unless the amp has been formally reviewed by Stereophile's John Atkinson or SoundStage!'s Bascom King (two publications that publish test-bench performance), you can build in an automatic safety buffer by insuring that your max playback SPLs remain well below the amplifier's power limits.

This isn't merely to avoid clipping but to stay in the amplifier's comfort zone where THD remains relatively stable and linear. Instead of attempting to do the math according to published speaker sensitivity, you can simply listen to your chosen SET/speaker combo and explore how much headroom you have outside of your normal listening levels. You do not want to habitually live just a few clicks below clipping. During your tests, play some large-scale symphonic fare and pay special attention to crescendos. Does tonality change? Does compression set in?

In general, pay attention to whether the bass seems properly damped so that as you descend into the midbass and lower bass, its gestalt doesn't change from tight and bouncy to loose and resonant. Listen for linearity so that it doesn't seem that different portions of the audible spectrum have different textures. If you transition from sharp to round back to sharp again, something's amiss. Listen for clarity. Excessive THD will sound ringy, indistinct, fuzzy, overly soft and perhaps even spiky, meaning tones in certain registers seem to stick out. Test this at different levels to make sure great results at lower levels remain constant at higher ones. Don't play complex music for this type of test so you aren't distracted by a multitude of voices or instruments but can clearly focus on one specific one and follow it
through volume changes as it climbs up and down the full breadth of its registers.

Granted, this still doesn't tell you whether the amp or speakers are to blame if you notice faults. After all, an inferior SET will misbehave even into SET-friendly loads. This is where research and knowledgeable dealers or friends can help you identify a known SET-friendly speaker which you can use as a gauge to assess a SET's overall quality. If the amplifier passes with a speaker known to be copasetic, then you can compare those results against the speaker you have your eyes on. Certain manufacturers of single-ended amps will list model and make of specific speakers they have personal experience with and can vouch for as being good matches.

In the end and far more so than with amps that keep doubling power down into 1 ohm and pride themselves on being uncompromisingly load invariant, successful results with SETs depend on experience and personal trial & error. Because they're an esoteric subset of HiEnd audio, the type of dealer selling such amplifiers tends to be far more of an enthusiast than hard-core corporate money man. After all, the latter always eyeballs the easy sell with the least amount of upfront hurdles to overcome and the least amount of potential after-sale maintenance or support issues. If you're in the market for SETs or simply curious to hear what they're about, consider patronizing an actual dealer who will be able to demonstrate whatever SETs he sells with the right kind of speaker. Do not put too much stock into casual Internet advice where people recommend Thiels and Magnepans and B&Ws for your purposes. They likely have no idea what they're talking about.

Simply put, SETs make the amplifier/speaker interface far more critical than usual. The only way to assure optimal results is due diligence. That means research and personal listening, two ingredients that used to be tantamount to assembling a superior HiFi rig: try before you buy. It's still as true as it was then - and perhaps nowhere as true as when it comes to putting together a micro to medium-power single-ended amplifier system. And before you counter that one could simply avoid the whole hassle by going after 60 to 100-watt SETs (which do exist though they're rare and usually very expensive), consider the strange truism that single-ended operation and the appetite for high power seem somewhat at odds if you wish to hear SET at its very best.

When low-power amps were the norm rather than exceptions (in the days
before transistors and pentodes), speaker designers still accounted for their particular requirements. It's only when solid-state power skyrocketed that speaker designers became lazy and got away with building unnecessarily hard-to-drive loudspeakers. Regardless of its popularity, the infamous Apogee Scintilla is a prime example of a hugely compromised design approach. If good sound is possible with a stable high impedance and narrow phase shifts, why promote a speaker that defies all common sense? It's like allowing your dog to crap indoors because you have a servant that will clean up after it. Many contemporary speakers crap on the floor expecting overbuilt amplifiers to deal with the mess. SETs are not that type of amp. They don't kowtow to belligerent speakers. SETs need speakers that serve them. Take care to honor that mandate if you're intent on hearing the luv uncut. Once you do, you might well feel like passing it around to spread that very infectious -- and addictive -- sickness.