Many audiophiles are drawn to valve power amplifiers or solid-state designs with low feedback because they prefer their sound. Subjective impressions of the superiority of such designs are often presented as being mysterious and wonderful, and almost beyond rational analysis.Yet some of the reasons may be fairly simple to explain, and can be exploited by anyone willing to try out what may seem like a weird idea to many conventional engineers.

OUTPUT IMPEDANCE
For some years I've felt that one aspect of this phenomenon that tends to be overlooked is that low feedback amplifiers, and ones with an output transformer 'outside the loop', tend to have a relatively high output impedance.
This then interacts with the speaker impedance, altering the overall tonal balance. Output impedances much above half an ohm or
so (a damping factor
of around 15 or less)
can produce audible
changes. And many
valve amps have a
higher output
resistance
than this.
With this in mind
I've been experimenting
with fitting series resistors to my speaker leads. The power amp I use is a high power solid-state design that takes the classic 'pure engineering' approach having a very low output impedance and a ruler-flat response.
So I can use resistors to dial in a chosen amount of output resistance and hear the effect.
The results have been quite interesting.

I tried the idea with a pair of Quad ESL 2805s. I love the Quads for their clear stereo imaging and natural sounding presentation of voice and acoustic instruments. But in my main listening room they tend to sound a bit bright in

the region around a few kHz. However, I'd noticed that the impedance of the 2805 takes a dip in the same frequency region. So I wondered if by adding some resistors I could tweak the sound to overcome this. processor' to modify the sound, even if it is a much less flexible method. I also used connectors so that I could quickly bypass the resistors to make comparisons.

RELAXED AND INVOLVING
My first reaction has been that the difference is an improvement. The sound does seem more relaxed and involving without losing imaging. Some material sounds too warm or woolly if I use too high a value of series resistor.
But I'm still experimenting. If you want to try something similar then I'd recommend choosing resistors that have a value in the

range below about 2.7ohm and power ratings that are at least 5W. Also choose ones said to have low inductance. And when trying them with loud music, make sure the resistors don't get too hot.

AT YOUR OWN RISK
What conclusion you draw will depend entirely on your tastes and equipment and there are some potential snags. One is that the level of distortion produced by the speakers may change. Another, of course, is that the change will make the sound worse, not better.
The resistors will also drop some of the signal voltage from your power amp, so you may have to wind up the volume control. You may also need to take care not to blow up your power amplifier by accidentally shorting its output.
You shouldn't try this experiment unless you know what I mean by 'in series' and how to add a series resistor to a wire. But some series resistance should be quite safe. If you decide you don't like the effect, then the cost is a lot less than having to modify your listening room or move house to get a better sound from your speakers! Maybe resistance isn't useless, after all.