I can think of a dozen links that discuss various measurements and their importance. But I can't recall exactly which one you mean. Most tube enthusiasts tend to say that high amounts of low order distortion aren't annoying and they place emphasis on high order artifacts. Some get into the proportions of various harmonics and what not.Personally, I'd expect that low amounts of distortion are best. But since speakers almost always generate over 1% second-harmonic content, I can see why maybe a tube amp making 0.5% might not be really noticable. Solid state amps can bring this down to 0.001% and figures like that.
As long as the amp isn't clipping, neither amp generates a lot of high orders, but when an amp is clipping, the transistor amp makes a sharp edge as the signal hits rail voltage, and that is equivalent to saying it generates multiple harmonics, including high order ones. A tube amp rounds the edges, so while the waveform produced is still artificial and distorted, it isn't quite a raspy sounding and harsh.
So my suggestion is, no matter what kind of amplifier you have, don't push it into clipping. 
Sorry I couldn't think of the specific URL you wanted. But here's one you might enjoy:
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