[xnor]

I think raising the output impedance is more important. If you want to add distortion the easy way google for tube simulation plugins.

 

[autoteleology]

I mean, you could replicate the low-order distortion of a tube with a solid state design, but you wouldn't be able to substitute the high-order distortion with the low-order distortion. Right?

 

[esldude]

Quote:

I mean, you could replicate the low-order distortion of a tube with a solid state design, but you wouldn't be able to substitute the high-order distortion with the low-order distortion. Right?

Well, good quality SS equipment will have distortion low enough though of some higher orders you can't hear it anyway.  So by adding the low order distortion similar to tubes that is what you will hear.
 
Using power amps on speakers, I have loaded the output of tube amps with power resistors, tapped it with a voltage divider and fed it on to a good SS amp.  What you got over the speakers driven by the SS amp was the sound of the upstream tube amp.  So low distortion, wide bandwidth SS amps are fully capable of replicating the sound of vacuum tubes.  The smooth, airy, spacious, 3D sound is all the result of various artifacts in tube power amps.  Perform processing to the source file in the digital world and one can replicate that tube sound without any tubes.
 
In Audacity there are several plug ins.  One is a harmonic generator.  You choose the level of harmonics to add to fundamentals for the 1st through the 10th harmonic.  Bumping up just the 3rd and 5th harmonic goes a long way to sounding like a PP triode.  There is more than just that however.  The output impedance on tube amps alters speaker response. And audible distortion tracks up and down with signal level in some tube amps while SS amps tend to stay low in distortion with an abrupt rise once you pass a certain signal level. 
 
It is all in principle something that can be replicated.  This company has made a business of emulating analog studio equipment digitally.
http://www.uaudio.com/uad-plug-ins.html

The videos are fairly informative about what they are doing.

 

[uchihaitachi]

When measuring source equipment, why do the measurements with load look inferior compared to measurements taken without load? How does load influence the ability of the source equipment to perform?

 

[stv014]

A low impedance load requires the amplifier to output higher current. This can increase the distortion in a number of ways, for example:
- the open loop output impedance is non-linear (negative feedback reduces, but does not entirely eliminate this), and higher current results in higher distortion voltage
- with higher currents/lower load impedance, the amount of some unwanted feedback effects increases (e.g. from power supply currents - which have high distortion - through ground loops or EMI; or the power supply voltage drops under heavy load, and this can be fed back into the audio output due to finite PSRR; also, thermal effects can cause low frequency distortion in some IC amplifiers)
- if the load impedance is low enough compared to the open loop output impedance, then it reduces negative feedback, although this is normally a minor issue compared to the others