Speakers for triode tube amps

OP

What some self-proclaimed "experts" obviously don’t understand is that when an amplifier starts asking for double the power by dropping from 8 to 4 ohms, it will start to distort if not designed for that level of output. There is no magical speaker, Tekton or otherwise, that can avoid this simple math if it’s a 4 ohm speaker by design...period. So a speaker that is playing a 1000hz note at 8ohms and using 15watts of power (say from a 25 watt MQ-88uSE amp) - suddenly hits a bass passage at 50hz and is 4ohms at that frequency now asks for 30 watts (double the 15), it will start to distort because it is operating beyond its design.

Don’t let anyone lead you astray....amp/speaker pairing is crucial. A high and flat impedance speaker is what you’re generally looking for with this Luxman amp.

btw - the Luxman MQ-88 u (SE or C) is the best executed KT-88 triode design I've ever heard.  It pairs wonderfully with DeVore and it really challenges what many of us tube snobs (myself included ;-)) think a KT-88 can sound like.  Many of the best attributes of push-pull and SET combined.  

Take a look here and you will find a number of very musical speakers that are easy to drive with higher output impedances (tube amps) and a bit less power, designed by Duke LeJeurne of Audiokinesis:

https://jamesromeyn.com/

The main concern with a speaker with a low nominal impedance, such as 4 ohm, and tube amps has to do with frequency response changes due to the relatively high impedance of the amp interacting with the impedance of the speaker.  While 4 ohms might be the nominal impedance, the actual resistance at each frequency is probably quite different, which means certain frequencies will be emphasized more than other depending on the speaker's actual resistance at that particular frequency.  If a 4 ohm rated speaker had a perfectly flat impedance curve (resistance at all frequencies the same) the impedance match of the speaker and the amp would not matter.  But, all speakers have a pretty wide ranging up and down curve regardless of their single number "nominal" rating.  Hence, all 4 ohm speakers are not the same when it comes to how they will react to the impedance of the amp as far as frequency response change.

Ideally, the amplifier's output impedance will be so low compared to the speaker impedance that there will be not much in the way of frequency response effects.  A tube amp with a 4 ohm tap will have enough enough of a turn ratio in the output transformer so as to lower the output impedance to make it work reasonably well with a 4 ohm speaker.  But, there is no free lunch, so this means a sacrifice of some power that can be delivered to the speaker.  If the amp only has an 8 ohm tap, it may not have a low enough output impedance to avoid major effects on the frequency response when working with a 4 ohm speaker.  Whether or not this effect is severe enough to disqualify the amplifier depends on the particular impedance curve of the speaker, the actual output impedance of the amp and the particular taste of the listener.  I've heard a number of supposed mis-matched speaker and amps that sounded quite nice.

Simple question, why not horn loaded speakers which seem to be a perfect match for low power tube amps?
As speakers they do tend be bigger size wise but small, low mass, fast, and efficient drivers easily match the attributes of a SET amp. The SET amp was developed to drive horn loaded speakers it wasn’t until much later that speaker design moved to a much smaller box adding much heavier drivers requiring much more power.

There are some high efficiency single driver open baffles but they usually require a sub.