Solid state design options...

Given all else being equal. All five of your parameters are all desirable goals for amp design.

Unfortunately all else are never equal. Designing andmanufactoring an amp at a given price range are always about compromises. That's why these goals are becoming more-or-less gimmicks instead. Every amp is always compromising something.

Unless the budget is astronomical, it's pretty unlikely to find optimize for all these goals.

Take a look at the statement amps from Levinson, Dynaudio, Krell, Classe, Musical Fidelity, Pass and etc should give you an idea. Even these guys don't have class A output if the rated output surpass 50-100 watts range.

The biggest class A rated output is probably Jadis 200 or the Pass Aleph series. Somewhere around 120W/ch, but they both have other issues as well.

To me the design parameters are important to me in the foll. order:

* Zero global feedback - reduces distortion & improves 3D imaging
* Class A output - for best linearity. We can have a class-A amp w/ lots of feedback but its worth very little!
* Instantaneous Current Delivery - this implies a very well designed power supply. Clayton Audio amps seem to rank the highest here. They are rated at 200A continuous! One could weld with them.
* High Damping Factor - this speaks about the amp output impedance. Since power amps are voltage sources, the best voltage source has zero output impedance. Of course, this is not physically possible so the next best is as low as possible over the entire audio band. Such amps will have killer bass response such as Symphonic Line & Wolcott tube amps. There are others, of course, but these come to mind right now.
* Balance In/Out - according to me this feature is the most hyped & most marketed! It has some advantages but not as much as what is hyped. I have no personal experience w/ BAT but this co. seems to make fully differential designs from input to output rather than create a single-ended design & then create an out-of-phase signal & call it balanced! Fully balanced designs seem to make sense if common-mode noise is an issue. I have heard single-ended designs that sound awesome so I'm not fully convinced that a balanced design brings a significant advantage to the table. Of course, JMO.

If only it were this simple... I have heard amps with negative feedback sound better than ones without negative feedback and vice versa. Same goes for class A versus AB and balanced versus single ended. As far as capacitance and dampening goes it really depends on the type of speaker you are using. As with most things design, execution and application are actually more important.

What I have found is that lower powered amps (including tube amps) generally sound better than higher powered amps in the same price range and if power is not an issue.

Ask the manufacturer of your speaker for amp recommendations.

How about the Belles 350A which is almost a purely class "B" design. It has a ultra high damping factor. Give one a listen if you get the chance or check out the review on Sounstage.com
Also, in the above discussion, a goodly number of amps use a cap to block DC on the input. This, to me, imparts a slight change in the sound for the worse. It seems input blocking caps have been substituted for DC servos (I know it's cheaper to do.) Also, as stated above, many amps go just a few watts into class A then crossover. With this, waveform distortions can occur. The positive and negative half cycles don't match up.
What i'm saying here is alot can happen to influence the sound of an amp besides the above listed things.
Why do people love tube amps---most of them stink on the test bench in any of these parameters. Most tube amps have a damping factor of around 20 or less! Could it be we like the "Distortions" a tube amp produces? (they sure test like crap!)

All the discussion points to one thing. Trade-offs for the typical audiophile amps. They were all optimized for a few particular things but not everything and they can't be due to cost consideration.

It's possible to design and build a 1000W/ch amp class A, balanced, zero-global feedback, balanced and high current delivery, but it will probably cost a quarter of a million dollars per channel and the size of a sofa.

So all these compromises designer made will basically force the marketing to emphasize specific strength which usually make all these terms unimportant and impossible to compare one on one.

If you are amp buyer, don't take these terms too seriously. If you are an amp designer, that's a complete different story.

BTW, if you can't tell yet, I am by training an eletrical engineer.