The top priced amps have a 0.03 THD while those below it are rated at 0.05%. The bottom tier one has 0.08%. Is there any discernable difference between the 0.03 and 0.05 THD??

Outlaw's distortion measurements are extremely conservative. Typical measurements I've taken on the Outlaw multichannel amps has resulted in distortion at 1kHz of .005% to .007% at full rated power - essentially an order of magnitude better than the stated spec.

In any event, distortion measurements are one of ones which small (or sometimes very large) differences is the distortion "numbers" means little or nothing. The difference between .03% and.05% is (if they did measure that high), is certainly insignificant, and not something you would ever hear the difference between.

Heck, vacuum tube power amplifiers can have as much as 1% distortion at full power! In spite of this, tube amplifiers are generally considered the gold standard in listening quality.

thanks! I never thought of that i.e. tube amps. I am still curious about those.

Knownalien, a tube buffer like the Yaqin at $175 (if you don't mind buying a Chinese product; a recent Absolute Sound says they're putting out a lot of tube stuff now) or a Musical Fidelity at about twice as much (if you'd rather not deal directly with Chinese brands), installed between preamp and power amp, will give you most of the "sweetness" and "musicality" of tube amps for a LOT less money. My Yaqin, bought before the Tibet troubles started, really works.

http://www1.epinions.com/content_312424107652 is what sold me on the Yaqin, but if you Google on "Yaqin tube buffer" you'll find lots of supporting comments. Maybe I'll upgrade the tube sometime but right now I'm very satisfied - even of the spec'd HD is 3-5 times as high as the amps you mentioned in your first question.
Re the audibility of HD in general, we need to remember two facts not often available: (1) high levels of odd-order HD make the sound irritating, while even-order HD simply sounds more "musical", even if there's a theoretical diminishment in perfect accuracy, but by high order I mean distortion levels of 2% or so; (2) what counts is am amp's HD at actually used rather than maximum output level; i.e., less than 5 watts - 500 milliwatts for high efficiency speakers like Klipsch or (yay!) Lowther. In fact for such speakers a half-watt feed from the amp would have you hollering "TURN IT DOWN SO I CAN HEAR MYSELF THINK!" This is not hyperbole.

My mains, at 106dB per watt efficiency fit into that category. In fact, the SET amplifier which I use for the high frequency horns is only rated at 5 watts (!), yet with those 5 watts I can produce something like 113dB above 500Hz alone.

One other thing about distortion - not only are the odd harmonics more irritating to the ear, but as the harmonic products go up in frequency, even very small (.00x%) amounts can color the sound of an amplifier or make it downright harsh. Harmonic components beyond 4th harmonic fall into this category. Some transistor amplifiers have significant amounts of distortion up to the 10th harmonic and beyond.

As noted, the really critical area is the first watt of output, because this is where we listen most of the time. It is in this area which not only harmonic distortion is a concern, but crossover distortion from less-than-optimum amounts of bias current flowing in the output stage can contribute a particularly harsh sounding distortion signature.

Because of the threat of thermal runaway, transistor amplifiers must transisition from class "A" to class "B" relatively early - in the first 5 watts or less. Tube power amplifiers by contrast are still operating in pure class "A" up to as much as half their total power output. That's a big reason, in addition to their more "musical" distortion signature, why they sound so listenable.

Knownalien,

To back up what psyprof1 suggested...Grant Fidelity also sells a tube buffer. It is the same make as the Yaqin, but may have a few minor changes circuitry wise (I am not absolutely sure). However, Ian Grant of Grant Fidelity will answer any questions you have, great costumer service. They sell for $215 and come with two tubes. There are few threads on Sound Thinking discussing the Music Fidelity X10, Yaqin, and the GF B283.

I bought two. I like them in my system. It provide the little extra that I was looking for without breaking my budget. For anyone with Solid State who wants to experiment or add a bit more musicality in their sound, great budget way to go. Plus, you can tube roll and tweak the sound.