This is the circuit of the crossover. Its more for a car with 8 ohm speakers than a 4 ohm iMac...
I would keep both tweeter capacitors 1 and 2 in circuit. They aren’t really big enough for 4 ohm tweeters as they cut down the mid frequencies too much. But the iMac tweeters are quite strident at about 3KHz so it may not be a bad thing. 😉
The inductor is too large (by about x2 - it will make the mid-frequency too quiet) for the tiny bass speaker in the iMac, so I would close that switch/jumper.
But the Woofer capacitor will cut down the bass response below about 200 Hz, so maybe too much. Or maybe it will only lose inaudible low bass, and will actually help the low bass you can hear.
Try it both ways?
You can test the polarity of the tweeter in the same way as the woofer, but you need slightly more voltage - just a quick dab of current so you can see it move and then disconnect it!
It's best to mark the speaker wire polarity before you remove the plug.
It goes Tweeter + - (blank blank) Woofer + -.
EDIT: Here's a guess at a frequency response plot of the crossover with the inductor (coil) in operation (no jumper B) and capacitor 3 (blue) by-passed by using jumper A. It's not accurate as I don't know the impedance of the inductor, but it's my best guess.
Both jumpers C & D are used to bring both smaller capacitors into operation.
Removing jumper A will affect the low bass (<200Hz) and not change the mid-range dip.
IF the speaker impedance is 8 ohm (twice the iMac speaker's) then the response curve will be more or less flat...
EDIT: Oct 2025. A Mk2 version of the crossover is being sold.
The Mk2 version is (for iMac use) electronically identical to the original one with ABCD jumper settings.
The blue bass-boost capacitor looks to be 22 microfarad in Mk2, but it is 100 microfarad in Mk1.
It is ONLY NEEDED when you are using the crossover with a subwoofer (like in a car)... 😵💫
To match the Mk 1 version, the #4 is identical to the Mk1 version with jumpers A,B,C and D connected.
This just uses the Tweeter capacitors, but removes the inductor (and the blue bass-boost capacitor) from the circuit.
Removing jumper B in Mk1 inserts the inductor, but with iMac 4 ohm speakers that may cut the midrange response.
To match that with Mk2 use this:
#5 cannot be matched with the Mk1 version.
Edit: Here's a quick doodle of the circuit:
