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Enter your amp and speakers, choose the stages you want, and get the resistor values for a JohnH reactive attenuator: the ideal value, the nearest standard value, how far off it is, and the watts each part has to handle.

Worked out in your browser from a circuit model. Nothing you enter leaves this page.

Your design, redrawn as you change it. Each stage switch is drawn engaged; its other throw ties the stage input to its output. The switch beside Rs4 closes in the lower first-stage position.

How the numbers are worked out

The amp is a source with the output impedance under Model settings: 2.5 × the speaker load unless you change it.
The speaker is a plain resistor of its rated impedance. A real speaker's impedance moves with frequency, so the dB you hear can differ a little from the model.
Attenuation and the loads the amp and speaker see are worked out at the check frequency.
Watts: the amp at its full rated power, a steady sine wave, through every setting from 20 Hz up to the top of the full-power band. Each part shows its worst case. Guitar signal carries far less power at the top end.
The part rating is the worst watts times your margin, rounded up to a common rating. The 2 × default is a suggestion, not a rule.