Well, having messed this up a little bit, you do need to know whether it's 20W into 8 ohms or 4 ohms or whatever. I do reccomend starting from the speaker side and for example (3 A)^2 * 8 = would be 72 Watts.
Agreed, to that you can't be supplying both the positive and the negative half cycle at the same time, so if you have +- rails, both cannot be used at full power at the same time.
The other problem is that speaker impeadence is not constant and you could have a Z of even 2 ohms at resonance.
I built an amp with +-50 V rails and 4 x 35 VAC at 3A each windings. I'm current limited in terms of power. I do use a 500 VA AC voltage regulator which improves the amp, but the AMP definately sounded better when I used a 20 A secondary Constant Voltage Trasformer. I had to abandon the latter because of transformer hum.