So do this at your own risk.
You will need:
Jacks £1.79 x 2 = £3.58 Matrix board £1.69 7w resistor 0.59 3w resistor 0.55 Altoids tin 0.90 Gym membership card - stolen Tools - already have some
£7.31 total, mostly from Maplin, easy to find stuff, that’s around 10 bucks. You could spend 100 dollars on something that does the same thing, but that’s no fun. It's a simple project, you'll make 10 solder joints, which isn’t too much work. Leaving out the chassis ground saves you 2 if you’re in a hurry.
The whole project took me a morning to build. Drilling was the slowest part for me, it helps if you have good tools. I had a small drill, a soldering iron, some wire, a multi-meter and a pair of pliers.
According to the attenuator calculator we need a 5.1 ohm 7w resistor in series with the speaker, and a 4.4 ohm 3w resistor in parallel for a 9db output cut to 8 ohm speakers. The values I ended up using were 5.6 and 4.7 ohms, the closest I could find at Malins. For a bigger amp, you’d need bigger resistors with the same vaules, a bigger tin and some way of getting rid of the heat. For a smaller amp, the extra beefy resistors won’t make a difference. A 5w amp like the Princeton will put out about a watt through this attenuator. If you're working with something like a Champ or an Orange AD5 find out the speaker impedance and do your own calculations.