What numbers are important in SS amps specs

FWIW - Watts ain't watts, you've got to know measurement conditions before even that measure is meaningful.
Anyway, your speaker manufacturer should provide you with a specification for minimum reccomended power & approximate nominal impedance. Once you have that in hand (e.g. 60 wpc continuous and 120wpc short term peak @ 4 ohms) then I'd reccomend looking at amps at and above that power range & trying to make the selection by listening to different amps paired to your speakers if at all possible.
It would be great if you could by the best sound just by looking at the numbers, but it doesn't work that way.

The number of times your foot taps the floor listening to it -- i.e. none!

As Jeff_Jones stated, you'd want to be concerned with how many watts are output into the load nominally presented by your speakers. When choosing my amp, I also looked closely at input impedance and power bandwidth. With SS amps THD will generally be less than 0.1%, so that's a spec that is often a wash, and not always that useful when stated by the manufacturer as just THD+Noise, without specifying full power, etc. Some audiophiles also make a big deal over slew rate, which tends to vary significantly between amps. I've owned high and low slew rate amps and liked both. Ultimately it is useful to check out what amps other owners of your speakers are using with them, and seeing how those are spec'ed, then just trust your ears...

The price. No price could be low enough for me to give up tubes.

Hbarrel, your very reasonable question is made complex by the experience many of us have had that none of the specifications are meaningful and consistent indicators of what an amp will actually sound like in real world listening experiences. The power output is useful ballpark information once you know the sensitivity of the speakers you plan to drive, and some specifications may tell you about a likely incompatibility (e.g., impedance at input and output). But in my experience, little to nothing in the way of specifications is a reliable indicator of what the unit will actually sound like.

For example, while it is certainly true that lower distortion is usually better than higher, an amp that has materially lower distortion achieved through the liberal application of negative feedback will most likely sound inferior to an amp with higher measured distortion but more judicious (or zero) use of negative feedback. It is also certainly true that higher slew rate capability (or "rise time", etc) can be a positive factor in achieving greater resolution and clarity, but high slew rate coupled with ringing and overshoot will sound terrible.

Over the years, I've given up looking at any specifications as indicators of good sound. If I need some indicator of what may be worthwhile auditioning, I instead consider build quality, robustness of power supply, parts quality and manufacturer's track record/reputation. Ultimately, I only care how the unit actually sounds when I listen to it and I refuse to even think about specifications as a substitute indicator of sound quality.
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