Can I power a 100w amplifier with a 33w PSU?
The datasheet says &quot;2 × 50 W Into a 4-Ω BTL Load at 21 V&quot; on the first page.
If your power supply outputs 19V you'll get 2x40W, because power scales as voltage squared.
If your speakers are 8 ohms instead of 4, then you'll get half the power, or 2x20W.
If you listen to music and not continuous sine waves, then your laptop adapter will probably be okay. If not, get a more powerful one from the recycling center, they're free.

Would I need to attenuate the amp to 33% to support the supplied power?

Power scales as square of voltage, so that would be square root of 0.33, or 0.57, but... no need.
If you have a recording that is pretty low in volume it's nice to be able to turn it up.
And if the amp draws too much power, the switching supply should current limit, so it'll just sound really bad. You'll notice.
Note: If this is for a subwoofer, then yeah, get another supply.