What's more important, watts or capacitance?

Look for the dynamic headroom and dynamic range spec (the loudest and softest sound it can play) many SS amps are over 100db. Look for high signal to noise ratio, some amps are 110. Look for high damping factor (this is what is supposed to "control" the speaker rather than the speaker controling the amp). Ignore THD spec, what really matters is THD + Noise. Look at the TDN+N over the power band - does the amp only spec good at one power lever and rather poor at other power output levels? Look for how long an amp can do its rattet power - often times the power rating is only for 30 seconds (If I recall the FTC requirements). Look at input impedance too - this is important for mating to the preamp- you want very high value like 100Kohms or more.

Slanski62-what speakers and power amps are you looking at?
this might be easier to ask as there maybe some agoners out there who have experience with what you are considering and can better answer your concerns with realtime feedback.
You may also see a response from Atmasphere on this question if Ralph does pay attention to his answer.

Thanks everybody for your helpful answers. My current amp is the Bryston 14B-SST2, which i use with the BAT VK42SE pre. I'm considering either the ARC HD220 or the BAT VK-600. My speakers are a custom design by Selah Audio. They're called the Visionarios and are shown on the site. My hypothesis is that with the Bryston, I'm not using it's 900W power output anyway, so maybe an amp with lower output but greater energy storage would represent an improvement.

I don't listen at real high volumes, but I do like classical music at close to live levels.

Classical music can get to 115 db in the first 5 rows of almost any concert hall. To do that with your speakers will require over 1000 watts.

A nimble amplifier- now that is another matter. It would be great if power supply filter capacitance and power were all that you needed to know, but its way more complicated by that! The one thing you need to know about this as that you cannot look at the spec sheets made by anyone and get a clear picture of the amp which will have the fastest transient response.

One spec that points to that though is risetime or slew rate. A slow risetime is 15Volts/microsec, a fast amplifier is over 100V (our amps, which are tube, have a 600V/usec risetime in the output section). You will find also the the shorter the signal path is in the amp or preamp, that there is a loose correlation to the 'speed' of the amp.

Feedback in the amplifier will slow its response, though quite often at the same time will seem to make the amplifier sound brighter (due to the addition of trace amounts of odd-ordered harmonics). Its one thing to have speed, but its another for the price to be one of irritation, so this is an area of concern for any designer!

Consequently, this really becomes a matter of audition.