G'day mate,
Good job on the Theos. Those specs are rated as power handling- input power. So it means this is the safe range that the Theos will handle, should you choose an amplifier anywhere between this power output range.
The most important thing to consider is not just power but high current and stable voltage. Electrostats have a notorious impedence swing, they're referred to as reactive loads. As the frequency changes so does the impedence and this can vary from high as 22 Ohms and as low as 0.7 Ohms. That's less than 1 Ohm and this swing takes place in a blink of eye!
This varying reactive load, is what causes many so called power amplifiers to crap themselves. They just don't have the required high current nor stable voltage to handle difficult loads, which is typical of Stats. Hence, power alone is not everything, in fact it doesn't mean anything!
I've used several so called high powered amplifiers to drive Maggie's, Quad ESL's and Apogee's, (without mentioning brand names) most of them simply shut down. They run out of puff and they're more suited to benign loads or much higher efficiency speakers, like Horns.
So the question is, how do you know what type of current or measurable range of current capacity the amplifier can produce? Well, two things you could look for:
1. Check if current values are stated in the specs, most high-end gear will indicate the specs as amps. This is a good indication that it can produce high current and hold steady voltage. Ideal for driving Stats effectively.
2. The power output ratings, sated as RMS power output into 8 Ohms, then doubles at 4 Ohms and triples at 2 Ohms. Ex. 50w/ch x 8 Ohms, 100w/ch x 4 Ohms, and 200w/ch x 2 Ohms. This basically means that the amplifier is stable all the way down to 2 Ohms.
There are other brands that will have specs at 1 Ohm, these are quite rare specs but again ideal for handling such impedence swings found in Stats.
If you're planning on getting a 20w integrated, just make sure it can handle down to 2 Ohms or at least doubles in power at 4 Ohms or below. If these specs aren't stated then it means this amp will blow up like the 4th of July trying to drive stats. It simply won't work.
Also look out for Class A or Class AB power ratings, as this is a very good indication of the amplifier's output stage.
Cheers and all the best,
RJ